audit(aws): surface OpenSearch RI tag-failure for ops monitoring (closes #250)

[cristim]

Summary

• Replace the freeform WARNING: log line in PurchaseCommitment with a
  structured OPENSEARCH_TAG_FAILED commitment_id=<id> error=<msg> line so
  operator dashboards and CloudWatch Logs Insights queries can alert on
  untagged RIs
• Add runbooks/opensearch-untagged-ri.md covering background (unsupported
  reserved-instance ARN type), manual re-tagging steps, and the fallback for
  when AWS still rejects the call
• Update the PurchaseCommitment comment to reference the runbook

Test plan

• TestPurchaseCommitment_TagFailure_StructuredLog (new): verifies
  OPENSEARCH_TAG_FAILED sentinel and commitment_id= field appear in the
  log, purchase result is still Success=true, and account ID is absent from
  the line
• Existing TestClient_PurchaseCommitment_AddTagsFailureDoesNotFailPurchase
  remains green (non-fatal contract unchanged)
• go vet ./providers/aws/services/opensearch/... clean

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: d9d6ae06-981a-433e-9662-a5e0d58c5d7f

📥 Commits

Reviewing files that changed from the base of the PR and between 4956d66 and c35a315.

📒 Files selected for processing (3)

• providers/aws/services/opensearch/client.go
• providers/aws/services/opensearch/client_test.go
• runbooks/opensearch-untagged-ri.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/250-wave10

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