feat(errors): surface rate-limit headers on HTTP 429 failures

[platinummonkey]

Summary

• Captures Datadog X-RateLimit-* response headers from typed SDK commands (via middleware), raw HTTP helpers, and pup api
• On HTTP 429, prints readable rate-limit details (rule name, limit, remaining, period, reset) to stderr with a retry hint
• Exits with code 429 so automation can distinguish rate-limit failures from other errors

Closes #747

Changes

• src/rate_limit.rs — header extraction, formatting, exit-code selection, and unit tests
• src/client.rs — RateLimitCaptureMiddleware on the SDK reqwest client
• src/raw_client.rs — HttpError.rate_limit field populated from response headers; integration test with mock 429
• src/main.rs — central CLI error handler prints enhanced 429 output and exits 429
• src/commands/api.rs — store rate-limit headers before failing non-2xx responses

Example output

Error: failed to list monitors: error in response: status code 429 Too Many Requests
Rate limit details:
  rule: get_all_monitors
  limit: 1000
  remaining: 0
Hint: rate limited — wait and retry

Test plan

• cargo test rate_limit
• cargo test test_raw_get_rate_limit_includes_headers
• cargo clippy -- -D warnings
• cargo fmt --check

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