Add channel test providers for Email, Slack, Teams, and Webhook

- Implemented EmailChannelTestProvider to generate email preview payloads.
- Implemented SlackChannelTestProvider to create Slack message previews.
- Implemented TeamsChannelTestProvider for generating Teams Adaptive Card previews.
- Implemented WebhookChannelTestProvider to create webhook payloads.
- Added INotifyChannelTestProvider interface for channel-specific preview generation.
- Created ChannelTestPreviewContracts for request and response models.
- Developed NotifyChannelTestService to handle test send requests and generate previews.
- Added rate limit policies for test sends and delivery history.
- Implemented unit tests for service registration and binding.
- Updated project files to include necessary dependencies and configurations.

docs/ARCHITECTURE_ATTESTOR.md

@@ -200,7 +200,8 @@ Indexes:
   * Predicate `predicateType` must be on allowlist (sbom/report/vex-export).
   * `subject.digest.sha256` values must be present and well‑formed (hex).
 * **No public submission** path. **Never** accept bundles from untrusted clients.
-* **Rate limits**: per mTLS thumbprint/license (from Signer‑forwarded claims) to avoid flooding the log.
+* **Client certificate allowlists**: optional `security.mtls.allowedSubjects` / `allowedThumbprints` tighten peer identity checks beyond CA pinning.
+* **Rate limits**: token-bucket per caller derived from `quotas.perCaller` (QPS/burst) returns `429` + `Retry-After` when exceeded.
 * **Redaction**: Attestor never logs secret material; DSSE payloads **should** be public by design (SBOMs/reports). If customers require redaction, enforce policy at Signer (predicate minimization) **before** Attestor.
 
 ---
@@ -233,6 +234,10 @@ Indexes:
 * `attestor.dedupe_hits_total`
 * `attestor.errors_total{type}`
 
+**Correlation**:
+
+* HTTP callers may supply `X-Correlation-Id`; Attestor will echo the header and push `CorrelationId` into the log scope for cross-service tracing.
+
 **Tracing**:
 
 * Spans: `validate`, `rekor.submit`, `rekor.poll`, `persist`, `archive`, `verify`.