fix(agents): Introduced a final transcription buffer for chatCtx fix

[affablebloke]

Fix incomplete messages in chatCtx introduced in #301

When the transcription forwarder was replaced with the text-audio synchronizer in PR #301, it introduced a regression where incomplete messages were being added to chatCtx. This PR resolves that by ensuring only finalized or properly buffered messages are pushed to the chat context.

Why:

We observed that chatCtx was receiving cut-off or partial transcriptions, often just the last sentence, which negatively affected the AI’s responses and also broke VPAEvent.AGENT_SPEECH_COMMITTED.

Discussion:

This raises a broader design question about interruptions and the agent’s chatCtx: Should the agent commit its full, uninterrupted message to chatCtx, or just the portion spoken before the interruption? In this PR, I opted to commit the interrupted message, but this is arguably a subjective design choice.

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