Components.AI: Define and enforce the conversation interaction-state matrix

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

Controls disagree about `ConversationStatus`: awaiting input is treated as idle, empty input can submit, and concurrent sends can race into exceptions.

### Describe the solution you'd like

Define enabled, disabled, busy, send, stop, and approval behavior for every conversation status and enforce it consistently across composer controls, suggestions, and actions. Invalid UI transitions must be guarded without silently ignoring invalid input.

### Additional context

Leaf task for #68227 requirements G2-G4 and G7-G8.

Start by reading parent issue #68227 requirements G2-G4 and G7-G8, then trace ConversationStatus usage across composer controls, suggestions, and actions. Done means each status has defined enabled, disabled, busy, send, stop, and approval behavior, with invalid transitions guarded and invalid input not silently ignored.

Written by the indexing model from the issue text.