Complete project health hardening roadmap

[wilsonfreitas]

Summary

Complete the project-health hardening roadmap from #44.

This PR consolidates the completed implementation work for:

• normalized HTTP error handling across SGS, Currency, and OData
• safer OData service-root, metadata, query, and string-filter handling
• aligned sync and async behavior for currency missing-symbol handling
• hardened currency response parsing and public input validation
• async client lifecycle API/documentation updates
• documentation and examples cleanup
• expanded lint, test, integration, docs, coverage, and dependency-audit gates

Verification

• uv run pytest -m "not integration": 185 passed, 11 deselected
• uv run pytest -m integration: 11 passed, 185 deselected
• uv run ruff check bcb/ tests/: passed
• uv run ruff format --check bcb/ tests/: passed
• uv run mypy bcb/: passed
• uv run --group docs sphinx-build -b html docs docs/_build/html: passed
• uvx pip-audit --requirement requirements-audit.txt: no known vulnerabilities found

Closes #44.