feat: Add in-memory implementations for issuer audit, key, repository, and trust management

- Introduced InMemoryIssuerAuditSink to retain audit entries for testing.
- Implemented InMemoryIssuerKeyRepository for deterministic key storage.
- Created InMemoryIssuerRepository to manage issuer records in memory.
- Added InMemoryIssuerTrustRepository for managing issuer trust overrides.
- Each repository utilizes concurrent collections for thread-safe operations.
- Enhanced deprecation tracking with a comprehensive YAML schema for API governance.

src/Api/StellaOps.Api.OpenApi/_shared/schemas/deprecation.yaml

@@ -0,0 +1,209 @@
+# Deprecation metadata schema for OpenAPI extensions
+# Used by API Governance tools for deprecation tracking and notification workflows.
+# Per APIGOV-63-001.
+schemas:
+  DeprecationMetadata:
+    type: object
+    description: |
+      Deprecation metadata for API endpoints. Applied as x-deprecation extension
+      on operation objects. Used by Spectral rules, changelog generation, and
+      notification templates.
+    required:
+      - deprecatedAt
+      - sunsetAt
+      - successorPath
+      - reason
+    properties:
+      deprecatedAt:
+        type: string
+        format: date-time
+        description: ISO 8601 timestamp when the endpoint was marked deprecated.
+        example: "2025-01-15T00:00:00Z"
+      sunsetAt:
+        type: string
+        format: date-time
+        description: ISO 8601 timestamp when the endpoint will be removed.
+        example: "2025-07-15T00:00:00Z"
+      successorPath:
+        type: string
+        description: Path to the replacement endpoint (if available).
+        example: "/v2/resources"
+      successorOperationId:
+        type: string
+        description: Operation ID of the replacement endpoint.
+        example: "getResourcesV2"
+      reason:
+        type: string
+        description: Human-readable explanation for the deprecation.
+        example: "Replaced by paginated v2 endpoint with cursor-based pagination."
+      migrationGuide:
+        type: string
+        format: uri
+        description: URL to migration documentation.
+        example: "https://docs.stella-ops.org/migration/resources-v2"
+      notificationChannels:
+        type: array
+        description: Notification channels for deprecation announcements.
+        items:
+          type: string
+          enum:
+            - slack
+            - teams
+            - email
+            - webhook
+        default:
+          - email
+      affectedConsumerHints:
+        type: array
+        description: Hints about affected consumers (e.g., SDK names, client IDs).
+        items:
+          type: string
+      breakingChanges:
+        type: array
+        description: List of breaking changes in the successor endpoint.
+        items:
+          $ref: '#/schemas/BreakingChange'
+
+  BreakingChange:
+    type: object
+    description: Description of a breaking change between deprecated and successor endpoints.
+    required:
+      - type
+      - description
+    properties:
+      type:
+        type: string
+        enum:
+          - parameter-removed
+          - parameter-renamed
+          - parameter-type-changed
+          - response-schema-changed
+          - header-removed
+          - header-renamed
+          - status-code-changed
+          - content-type-changed
+          - authentication-changed
+        description: Category of the breaking change.
+      path:
+        type: string
+        description: JSON path to the affected element.
+        example: "$.parameters[0].name"
+      description:
+        type: string
+        description: Human-readable description of the change.
+        example: "Parameter 'page' renamed to 'cursor'"
+      migrationAction:
+        type: string
+        description: Recommended action for consumers.
+        example: "Replace 'page' parameter with 'cursor' using the nextCursor value from previous response."
+
+  DeprecationNotificationEvent:
+    type: object
+    description: Event payload for deprecation notifications sent to Notify service.
+    required:
+      - eventId
+      - eventType
+      - timestamp
+      - tenantId
+      - deprecation
+    properties:
+      eventId:
+        type: string
+        format: uuid
+        description: Unique identifier for this notification event.
+      eventType:
+        type: string
+        const: "api.deprecation.announced"
+        description: Event type for routing in Notify service.
+      timestamp:
+        type: string
+        format: date-time
+        description: ISO 8601 timestamp when the event was generated.
+      tenantId:
+        type: string
+        description: Tenant scope for the notification.
+      deprecation:
+        $ref: '#/schemas/DeprecationSummary'
+
+  DeprecationSummary:
+    type: object
+    description: Summary of a deprecated endpoint for notification purposes.
+    required:
+      - service
+      - path
+      - method
+      - deprecatedAt
+      - sunsetAt
+    properties:
+      service:
+        type: string
+        description: Service name owning the deprecated endpoint.
+        example: "authority"
+      path:
+        type: string
+        description: API path of the deprecated endpoint.
+        example: "/v1/tokens"
+      method:
+        type: string
+        enum:
+          - GET
+          - POST
+          - PUT
+          - PATCH
+          - DELETE
+          - HEAD
+          - OPTIONS
+        description: HTTP method of the deprecated endpoint.
+      operationId:
+        type: string
+        description: OpenAPI operation ID.
+        example: "createToken"
+      deprecatedAt:
+        type: string
+        format: date-time
+      sunsetAt:
+        type: string
+        format: date-time
+      daysUntilSunset:
+        type: integer
+        description: Computed days remaining until sunset.
+        example: 180
+      successorPath:
+        type: string
+        description: Path to the replacement endpoint.
+      reason:
+        type: string
+        description: Deprecation reason.
+      migrationGuide:
+        type: string
+        format: uri
+      changelogUrl:
+        type: string
+        format: uri
+        description: URL to the API changelog entry for this deprecation.
+
+  DeprecationReport:
+    type: object
+    description: Aggregated report of all deprecations for changelog/SDK publishing.
+    required:
+      - generatedAt
+      - schemaVersion
+      - deprecations
+    properties:
+      generatedAt:
+        type: string
+        format: date-time
+        description: When this report was generated.
+      schemaVersion:
+        type: string
+        const: "api.deprecation.report@1"
+      totalCount:
+        type: integer
+        description: Total number of deprecated endpoints.
+      upcomingSunsets:
+        type: integer
+        description: Number of endpoints with sunset within 90 days.
+      deprecations:
+        type: array
+        items:
+          $ref: '#/schemas/DeprecationSummary'