fix(anthropic): map webp images explicitly

[he-yufeng]

Fixes the Anthropic WebP part of #2204.

The Anthropic formatter was relying on mimetypes.types_map for image media types. That map is platform/Python-install dependent, so .webp can fall back to application/octet-stream even though Strands already accepts webp as an image format.

This keeps a small explicit map for the Anthropic image formats Strands supports and falls back to mimetypes only for unknown formats.

Validation:

• python -m pip install -e ".[anthropic]"
• python -m pytest tests/strands/models/test_anthropic.py -q
• python -m ruff check src/strands/models/anthropic.py tests/strands/models/test_anthropic.py
• python -m ruff format --check src/strands/models/anthropic.py tests/strands/models/test_anthropic.py
• git diff --check

[he-yufeng]

Rebased this onto current main and revalidated the Anthropic model path. Current head is 2ba48cb. Validation from strands-py: uv sync --extra dev --extra anthropic; uv run pytest tests/strands/models/test_anthropic.py -q --basetemp ...tmp\pytest-2304 -p no:cacheprovider: 58 passed; uv run ruff check src/strands/models/anthropic.py tests/strands/models/test_anthropic.py; uv run ruff format --check src/strands/models/anthropic.py tests/strands/models/test_anthropic.py; python -m py_compile strands-py\src\strands\models\anthropic.py strands-py\tests\strands\models\test_anthropic.py; git diff --check origin/main...HEAD.

[codecov]

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