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