feat(bedrock): allow selecting Mantle auth mode

[mikemikimike]

Summary

Adds an explicit auth_mode option to AnthropicBedrockMantle and AsyncAnthropicBedrockMantle.

• auto preserves the existing authentication precedence.
• api_key selects bearer authentication even when AWS credentials are provided.
• sigv4 selects the refreshable AWS credential chain even when bearer-key environment variables are set.
• copy() and with_options() preserve or override the selected mode.

This gives applications using ECS task roles, EC2 instance profiles, IRSA, or other rotating AWS credentials a public way to avoid ambient bearer-key selection.

Closes #1893

Tests

• python -m pytest tests/lib/test_bedrock_mantle.py -q — 39 passed
• ruff check src/anthropic/lib/aws/_credentials.py src/anthropic/lib/bedrock/_mantle.py tests/lib/test_bedrock_mantle.py
• ruff format --check on the changed files
• pyright src/anthropic/lib/aws/_credentials.py src/anthropic/lib/bedrock/_mantle.py
• git diff --check

The test run uses the checkout's src via PYTHONPATH and does not require AWS credentials or network calls.