What does this PR do?

This PR adds model fallback support for agents. When a primary model fails or is too slow to respond, the system automatically cycles through configured fallback models.
feat #7602 #9575

Key changes:

• New config options: fallback_models (list of fallback models in priority order) and first_token_timeout (base timeout in ms for first-token detection, scaled by input size: base + inputChars * 0.5)
• Fallback cycling logic (SessionFallback): builds a deduplicated model list, cycles through all models starting from the last successful one, and remembers which model worked per session/agent pair
• TTFT timeout: measures time-to-first-token from after LLM.stream() returns (excluding framework init overhead), so the timeout reflects actual network/model latency
• Short retry before switching: for retryable API errors, one retry attempt is made on the same model before falling back to the next
• Error handling: context overflow and user abort errors skip fallback (not model-specific); FirstTokenTimeoutError is properly unwrapped from AbortError so it triggers fallback instead of
  being treated as user cancellation
• Toast notification: when a fallback switch occurs, a toast is shown indicating the model change
• Edge case fixes: clamps out-of-bounds fallbackStartIndex to prevent infinite loops when models are removed from config; resets both indices when the remembered model fails to resolve

Files changed:

• src/session/fallback.ts — new module for fallback state management and model cycling
• src/session/processor.ts — main integration: TTFT timer, fallback loop, error classification
• src/agent/agent.ts / src/config/config.ts — schema additions for fallback_models and first_token_timeout
• src/session/prompt.ts / src/session/message-v2.ts — pass fallback config to processor; handle FirstTokenTimeoutError

How did you verify your code works?

• Added unit tests for SessionFallback (buildModelList, nextIndex cycling, recordSuccess/getStartIndex, full cycle simulations including remembered-model and out-of-bounds edge cases) — see test/session/fallback.test.ts
• Added unit tests for TTFT computation (estimateInputChars, computeTtftTimeout with various input sizes) and FirstTokenTimeoutError serialization — see test/session/processor-ttft.test.ts
• Manual testing with multiple model configurations to verify fallback cycling, TTFT timeout triggering, and toast notifications