feat(client): support callable auth tokens

[wilyan09007]

Summary:
Fixes #1496 by allowing auth_token to be a string or a callable returning the current bearer token. Callable tokens are resolved when auth headers are built, so long-running clients can use refreshed tokens.

Changes:

• Add an AuthToken alias for static and callable bearer tokens.
• Resolve callable auth tokens per request for sync and async clients.
• Preserve callable token behavior through copied clients.
• Document short-lived bearer token usage in the README.
• Add tests for callable tokens and credential precedence.

Testing instructions:

• Run uv run python -m pytest tests\test_client.py -k "auth_token_callable or validate_headers" -n0.
• Run uv run python -m pytest tests\lib\test_credentials.py -k "auth_token_callable or explicit_auth_token_shadows or copy_with_explicit_api_key_shadows" -n0.
• Run uv run ruff check src\anthropic\_client.py src\anthropic\__init__.py tests\test_client.py tests\lib\test_credentials.py README.md.
• Run uv run pyright src\anthropic\_client.py src\anthropic\__init__.py.