feat(tools): add async token-bucket rate limiter middleware for tool execution

[glatinone]

This PR introduces an opt-in Async Token-Bucket Rate Limiter and resilience middleware for tool execution:

• New middleware: api/core/tools/middleware/rate_limiter.py (RPM/TPM configurable, async-safe)
• Tool engine integration to acquire tokens before outbound calls
• Exponential backoff with jitter on HTTP 429/503 and respect Retry-After
• Unit tests: api/tests/unit_tests/core/tools/middleware/test_rate_limiter.py

Backwards compatible and disabled by default; enables safer multi-agent concurrency and reduces upstream 429s.

[glatinone]

Closing this PR to avoid adding unnecessary review overhead for maintainers. Thank you for your time!