Align async currency missing-symbol behavior

[wilsonfreitas]

Summary

• keep the existing sync missing-symbol contract as the shared contract
• make async currency calls skip only CurrencyNotFoundError results in mixed requests
• preserve propagation for non-not-found API, parsing, and HTTP failures
• use a clearer all-invalid symbol error message
• add sync/async tests for mixed, all-invalid, duplicate, dataframe, and text cases

Fixes #42

Verification

• uv run pytest -m "not integration" (143 passed, 11 deselected)
• uv run ruff check bcb/ tests/
• uv run ruff format --check bcb/ tests/
• uv run mypy bcb/
• uv run --group docs sphinx-build -b html docs docs/_build/html