🤖 Fix assistant message part timestamps

[ammar-agent]

Problem

Assistant message timestamps were inconsistent across reloads:

• Before reload: All parts showed the timestamp of the first message
• After reload: All parts showed the timestamp of the last message (or Date.now())

This happened because:

1. Text/reasoning parts weren't getting timestamps during streaming (unlike tool parts)
2. Display logic used message-level timestamp instead of part-level timestamps
3. Reconnection case was overwriting metadata.timestamp with Date.now()

Solution

• Add timestamp field to text/reasoning parts during streaming (matching tool parts)
• Preserve first timestamp when merging adjacent parts of the same type
• Use part.timestamp ?? baseTimestamp when displaying (consistent with tool parts)
• Preserve metadata.timestamp in reconnection case instead of overwriting

Testing

Added comprehensive tests covering:

• Timestamp assignment during streaming
• Timestamp preservation when displaying
• Fallback to message-level timestamp for old history without part-level timestamps

All existing tests pass.

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