#299 fix(broadcasts): add topic_id to the Broadcast response type

Type checkers now accept topic_id when you read a broadcast. The API already returns the field, but the response type did not declare it, so strict mypy rejected the key.

Not covered. Six other broadcast response fields can be null, but the SDK still types them as strings. A nullable type can break type checks in user code, so that change needs its own release decision.

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Summary by cubic

Declares topic_id on the Broadcast response type so strict type checkers accept the field. The API already returns it, so there's no runtime behavior change.

Not covered

• Six other broadcast response fields can be null but are still typed as strings, which can break type checks in user code.