Add rtstream namespace support to Collection.search

[ashish-spext]

Motivation

• Extend collection-level search to support RTStream results returned by a new collection-scoped rtstream search endpoint.
• Allow callers to opt into RTStream-style searches via a namespace parameter and pass RTStream-specific options.
• Return RTStream search result objects instead of video SearchResult when the rtstream namespace is used.

Description

• Added new parameters to Collection.search including namespace, scene_index_id, stitch, rerank, rerank_params, and index_platform, and updated the return type to Union[SearchResult, RTStreamSearchResult].
• When namespace == "rtstream" the method now builds a payload from the provided args and posts to the new endpoint f"{ApiPath.rtstream}/{ApiPath.collection}/{self.id}/{ApiPath.search}".
• Maps the response results into RTStreamShot objects and returns an RTStreamSearchResult(collection_id=self.id, shots=shots).
• Kept existing behavior for non-RTStream searches by delegating to SearchFactory and existing search implementations.

Testing

• No automated tests were run for this change.

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Codex Task