fix(bedrock): normalize 3gp video format

[he-yufeng]

Summary

Fixes the Bedrock 3GP part of #2204.

Bedrock expects the video format enum to be three_gp, while common MIME/extension paths produce 3gp, 3gpp, or 3g2. The SDK currently forwards the value as-is, so a valid 3GP video content block can be rejected by the Bedrock Converse API.

This normalizes those aliases when formatting Bedrock video content blocks. Other media-format gaps from #2204 are left out of this PR.

To verify

Ran locally:

python -m pytest tests/strands/models/test_bedrock.py::test_format_request_maps_3gp_video_formats -q
python -m pytest tests/strands/models/test_bedrock.py -q
python -m ruff check src/strands/models/bedrock.py tests/strands/models/test_bedrock.py
python -m py_compile src/strands/models/bedrock.py tests/strands/models/test_bedrock.py
git diff --check

[he-yufeng]

Rebased this onto current main and revalidated the Bedrock model path. Current head is 444de41. Validation from strands-py: uv run pytest tests/strands/models/test_bedrock.py -q --basetemp ...tmp\pytest-2306 -p no:cacheprovider: 169 passed; uv run ruff check src/strands/models/bedrock.py tests/strands/models/test_bedrock.py; python -m py_compile strands-py\src\strands\models\bedrock.py strands-py\tests\strands\models\test_bedrock.py; git diff --check origin/main...HEAD. Note: ruff format --check wants to reformat pre-existing lines in origin/main's bedrock.py as well, so I did not include unrelated formatting churn in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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