Fix streaming top-level objects without list wrapping

[selamw1]

Description

A2UI v0.9 streams use consecutive top-level JSON objects (JSON Lines format) instead of wrapping them in a top-level array [...]. Previously, A2uiStreamParser skipped every character until it saw a [ bracket, causing all v0.9 blocks to be silently discarded.

Fixes #1527

This PR refactors the streaming parser's guard to use a fall-through design:

1. Allows the parser to recognize top-level objects (starting with { when brace count is 0) directly.
2. Simplifies the code by letting the standard loop handle brace counting and buffer/stack updates instead of duplicating them.
3. Adds tests in test_streaming_v09.py to ensure single and consecutive top-level objects are parsed successfully without list wrapping.

Pre-launch Checklist

• I signed the [CLA].
• I read the [Contributors Guide].
• I read the [Style Guide].
• I have added updates to the [CHANGELOG].
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

[gemini-code-assist]

Code Review

This pull request updates the streaming parser to support parsing single and multiple top-level JSON objects without array wrapping, and adds corresponding unit tests. Feedback suggests simplifying a redundant boolean condition in the parser logic to improve readability, and keeping the validator enabled in the new tests to maintain realistic test coverage.

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