- Removed the Google Interactions transport and deleted interaction-specific request options from the shared AI stream typing/API surface.
- Simplified Google provider routing to eliminate interactions auto-selection logic and keep `streamGoogle` on the `:streamGenerateContent` path.
- Updated Vertex request handling to use resolved stream hosts without `/interactions`/`Api-Revision` and removed related interaction constants.
- Deleted obsolete Interactions tests and updated remaining Google stream tests to no longer reference `useInteractionsApi`/`storeInteraction`/`previousInteractionId`.
- Added comprehensive tests for downshifting model behavior, including plan nudge injection and completion safety mechanisms.
- Verified session persistence accuracy by validating that from-disk rebuilds match the live agent state.
- Confirmed correct cache key propagation during tan commands to ensure provider caching is preserved.
- Removed obsolete reasoning slide tests.
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