Update the streaming parser to support A2UI spec v0.9

[nan-yu]

Description

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Pre-launch Checklist

• I signed the CLA.
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• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.

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[gemini-code-assist]

Code Review

This pull request refactors the A2UI streaming parser to enhance support for both v0.8 and v0.9 message formats, particularly for data model updates and component handling. The changes introduce an abstract A2uiVersionHandler with version-specific implementations for message recognition, surface ID extraction, component wrapping, and data model parsing. The A2uiStreamParser now utilizes these handlers, leading to a more modular and extensible design. Key updates include a new _get_loading_placeholder helper, refined JSON buffer processing, granular v0.9 data model update handling with path-based merging, and improved component dependency tracking. Comprehensive new tests for v0.9 functionality have been added. Review comments indicate a duplicate test function in test_streaming_v09.py, an unused parameter in _get_loading_placeholder, a potentially redundant or problematic lstrip call in _process_json_chunk, a commented-out debug log that should be addressed, and two TODOs in version_handlers.py highlighting architectural debt related to data model parsing and path depth support.

[nan-yu]

This needs to rebase once #991 is merged.