Components.AI: Scope streaming renders to the changing message

### Is your feature request related to a problem? Please describe the problem.

Every streaming block update currently re-renders the full message list, causing unnecessary work and unstable DOM behavior.

### Describe the solution you'd like

Update the rendering boundaries so only the affected message/block re-renders during streaming. Already completed turns and messages must retain component and DOM identity.

### Additional context

Leaf task for #68227 requirements A3 and J6. Requires stable turn/message/block identity.

Start by reading requirements A3 and J6 in #68227, then locate the Components.AI streaming render path. Trace how turn, message, and block identity are handled during updates. Done means only the changing message or block re-renders while completed turns and messages retain component and DOM identity.

Written by the indexing model from the issue text.