fix: decode JWT video grants from the camelCase claim keys

[RaphaelFakhri]

Summary

Fixes #1236.

LiveKitVideoGrant mapped its fields to snake_case keys such as room_join, can_publish and can_publish_sources. LiveKit tokens write the video claim with camelCase keys (roomJoin, canPublish, canPublishSources), as defined in livekit/protocol auth/grants.go and used by the server SDKs and the Android SDK. As a result, TokenSourceResponse.jwtPayload?.video returned null for every grant except room, hidden and recorder when it decoded a real token.

This change:

• Removes the snake_case @JsonKey renames from LiveKitVideoGrant, so json_serializable uses the field names, which match the claim keys.
• Regenerates jwt.g.dart.

Test plan

• test/token/token_source_test.dart: the parses claims and grants from token test and the _generateToken helper now build the video claim with the keys that LiveKit tokens use, and the test also checks canSubscribe.
• Before the change the test fails with Expected: true, Actual: <null> on grant.roomJoin. After the change it passes.
• flutter test test/token/, flutter analyze, dart format --set-exit-if-changed and import_sorter pass.
• Added a .changes entry.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.