feat: add opt-in retries honoring Retry-After

[mikemikimike]

Summary

• add shared, deterministic retry policy helpers for integer and HTTP-date Retry-After values
• add opt-in max_retries=0 support to sync and async clients
• retry safe endpoints for transient statuses and connection errors
• restrict crawl and non-stream research retries to HTTP 429 to avoid duplicate jobs
• leave streaming research behavior unchanged
• bound retry sleeps by the request timeout budget

Closes #177

Tests

• python -m pytest -q — 102 passed
• python -m compileall -q tavily
• git diff --check

No API keys, GPU, model downloads, or external services are required.