fix(rest/nodejs): retry transient webhook failures

[FanouZeng-TT]

Description

notifyWebhook treated every resolved fetch call as a successful delivery, so a 5xx response was silently dropped. Transport failures were logged once without retrying, even though the same event could be delivered safely with its original identity.

Fix: retry transport errors and 5xx responses up to three times with exponential backoff. Permanent 4xx responses are not retried, and all attempts reuse the same Webhook-Id, Webhook-Timestamp, and serialized body.

Category (Required)

• Core Protocol
• Governance/Contributing
• Capability
• Documentation
• Infrastructure
• Maintenance
• SDK
• Samples / Conformance
• UCP Schema
• Community Health (.github)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and Code of Conduct.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove the fix is effective.
• New and existing unit tests pass locally with my changes.
• I have included/updated the relevant JSON schemas (Core/Capability only).
• I have regenerated Python Pydantic models (not applicable).

Screenshots / Logs (if applicable)

• npm test: 123 passed
• npm run build: passed
• pre-commit run --all-files: passed