logger: retry transient Slack webhook failures

[chrismaree]

What Changed

• Added bounded retry handling around Slack webhook posts in packages/logger/src/logger/SlackTransport.ts.
• Retries Slack 429 responses and common transient 5xx statuses before surfacing a TransportError.
• Honors Slack Retry-After headers when present and caps the wait at 5 seconds to avoid long logger stalls.
• Added focused unit coverage for retry success, retry exhaustion, non-retryable failures, and Retry-After capping.

Why

• The PagerDuty incident was caused by Slack returning HTTP 429 to a normal monitor notification.
• Before this change, any Slack post failure immediately became a TransportError, which can be routed into PagerDuty when logTransportErrors is enabled.
• Retrying transient Slack delivery failures gives rate limits and short Slack-side outages a chance to clear before the logger emits a transport failure.
• Non-transient webhook errors, such as malformed payloads or revoked/bad webhook responses, still fail fast so real delivery misconfiguration remains visible.

Impact

• Reduces noisy PagerDuty incidents caused by transient Slack webhook rate limiting.
• Keeps existing Slack routing behavior and payload formatting unchanged.
• Keeps the final failure mode unchanged after retry exhaustion: the logger still returns a Slack TransportError to the existing transport-error path.
• Does not include deployment env values or webhook URLs.

High risk Sections to review with detail

• postWithRetry in SlackTransport.ts: retry count, retryable status list, and delay cap are the main behavior changes.
• Split-message Slack payload flow: each chunk now uses the same retry helper, so long messages may retry per chunk.
• getSlackPostRetryDelaySeconds: validates both numeric and date-form Retry-After headers while bounding delay time.

Validation

• Ran git diff --check and git diff --cached --check successfully.
• Did not run builds or tests, per repo instructions not to run them unless explicitly requested.
• Added tests in packages/logger/test/logger/SlackTransport.retries.js covering the intended retry behavior.

Docs

• No repo docs update was needed; this package does not have checked-in logger operational docs for Slack webhook retry policy or the managed OO monitor deployment env.