feat(bedrock): add explicit Mantle auth mode

[00200200]

Summary

• add auth_mode="auto" | "api_key" | "sigv4" to AnthropicBedrockMantle and AsyncAnthropicBedrockMantle
• allow callers to force SigV4 while ignoring ambient bearer-key environment variables
• preserve and switch the selected mode through copy() / with_options()
• validate missing API keys and conflicting skip_auth / auth_mode settings
• add sync and async regression coverage

Fixes #1893

Verification

• uv run pytest tests/lib/test_bedrock_mantle.py -q — 41 passed
• ./scripts/test — Python 3.10/Pydantic v2: 6026 passed; Python 3.10/Pydantic v1: 5813 passed; Python 3.14/Pydantic v2: 6026 passed; MCP v2: 39 passed
• uv run ruff check ... — passed
• uv run --all-extras pyright --project . — 0 errors
• uv run mypy src/anthropic/lib/bedrock/_mantle.py tests/lib/test_bedrock_mantle.py — passed

[gouveags]

This addresses the issue: callers can force SigV4 while retaining the default AWS credential chain, and the mode survives client copies. All 41 Mantle tests passed locally; additional local signing checks passed for sync and async clients, including copy() and with_options().

[gouveags]

Thanks for working on this! The implementation looks great to me, and I left an approval after reviewing and testing it. @craigie-ant could you please take a look when you have a chance?