feat: enable concurrent surface support by tracking root IDs per surface

[nan-yu]

This handles cases when parsing multiple A2UI beginRendering payloads back-to-back in the same token stream (as seen in multi_surface.json), the parser mistakenly overwrites the internal root_id tracker.

With the new change, the streaming parser is able to cleanly interpret interleaved UI topologies in real-time gracefully.

Description

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Pre-launch Checklist

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[gemini-code-assist]

Code Review

This pull request updates the streaming parser to support multiple concurrent surfaces by replacing the single _root_id with a mapping of surface IDs to root IDs and introducing an _unbound_root_id for cases where the root is identified before the surface. Review feedback identifies a regression in handling default root IDs for protocol versions like v0.9 and points out a bug in the metadata sniffing logic where the persistence of surface_id across messages can lead to incorrect root associations.