Add test coverage for `update_stream_metadata()` / `get_stream_metadata()`

[Austinaminu2]

File: contracts/streaming/src/lib.rs:1130-1155
Zero test coverage currently (confirmed via grep across all test*.rs files). Add tests covering: successful metadata update, reading it back, and the authorization/state checks the function already enforces.

[wealth4ever123]

I would like to take ownership of this issue. My experience includes building scalable systems, improving developer tooling, and implementing secure backend services with comprehensive test coverage.

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #465)
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