Add per-request timeout support

[wilsonfreitas]

Summary

• Add a shared request-timeout type/helper that preserves the existing client default when timeout is omitted.
• Expose keyword-only timeout support across SGS, currency, and OData sync/async request paths.
• Document timeout usage and add focused tests proving the value is passed to HTTP clients without changing global defaults.

Fixes #59

Verification

• uv run pytest tests/test_request_timeout.py tests/test_sgs_negative.py tests/test_currency.py tests/test_odata.py tests/test_async.py
• uv run ruff check bcb/ tests/
• uv run ruff format --check bcb/ tests/
• uv run mypy bcb/
• uv run pytest -m "not integration"
• uv run --group docs sphinx-build -b html docs docs/_build/html
• uv run pytest -m integration
• git -c core.whitespace=blank-at-eol,blank-at-eof,space-before-tab,cr-at-eol diff --cached --check