Implement ledger metrics for observability and add tests for Ruby packages endpoints

- Added `LedgerMetrics` class to record write latency and total events for ledger operations.
- Created comprehensive tests for Ruby packages endpoints, covering scenarios for missing inventory, successful retrieval, and identifier handling.
- Introduced `TestSurfaceSecretsScope` for managing environment variables during tests.
- Developed `ProvenanceMongoExtensions` for attaching DSSE provenance and trust information to event documents.
- Implemented `EventProvenanceWriter` and `EventWriter` classes for managing event provenance in MongoDB.
- Established MongoDB indexes for efficient querying of events based on provenance and trust.
- Added models and JSON parsing logic for DSSE provenance and trust information.

offline/notifier/templates/attestation/tmpl-attest-verify-fail.slack.en-us.template.json

@@ -0,0 +1,16 @@
+{
+  "schemaVersion": "notify.template@1",
+  "templateId": "tmpl-attest-verify-fail-slack-en-us",
+  "tenantId": "bootstrap",
+  "channelType": "slack",
+  "key": "tmpl-attest-verify-fail",
+  "locale": "en-us",
+  "renderMode": "markdown",
+  "format": "slack",
+  "description": "Slack alert for attestation verification failures with Rekor traceability.",
+  "body": ":rotating_light: {{attestation_status_badge payload.failure.status}} verification failed for `{{payload.subject.digest}}`\nSigner: `{{fingerprint payload.signer.kid}}` ({{payload.signer.algorithm}})\nReason: `{{payload.failure.reasonCode}}` — {{payload.failure.reason}}\nLast valid attestation: {{link \"Console\" payload.links.console}}\nRekor entry: {{link \"Transparency log\" payload.links.rekor}}\nRecommended action: {{payload.recommendation}}\n",
+  "metadata": {
+    "author": "notifications-bootstrap",
+    "version": "2025-11-12"
+  }
+}