fix: add 429 rate limit retry with exponential backoff

[xodn348]

Fixes #812.

This makes transient 429 rate limit failures recoverable for agent sessions:

• adds rate_limit_max_retries to ClaudeAgentOptions with a default of 3 and 0 as an opt-out
• retries only subprocess failures that include 429 / rate_limit_error
• uses retry delay from the error payload when available, otherwise exponential backoff with jitter
• captures subprocess stderr so ProcessError carries the API error payload needed for retry detection
• raises RateLimitError after retry exhaustion with the original ProcessError attached

I rebased this on current upstream/main and kept the retry path scoped to rate-limit process errors.

Testing:

uv run --extra dev pytest -q tests/test_rate_limit_retry.py
uv run --extra dev ruff check src/claude_agent_sdk tests/test_rate_limit_retry.py
uv run --extra dev mypy src/claude_agent_sdk tests/test_rate_limit_retry.py
python3 -m compileall -q src/claude_agent_sdk tests/test_rate_limit_retry.py

Results: 14 targeted tests passed; ruff passed; mypy passed; compileall passed.