Problem

When switching models (via /model or fallback), AIAgent._config_context_length is never cleared, so the new model inherits the previous model's context window instead of auto-detecting the correct one via get_model_context_length().

Root Cause

AIAgent._config_context_length is set once during __init__ from model.context_length in config.yaml. This value is never cleared in either:

1. switch_model() — the /model command handler
2. _try_activate_fallback() — the failover path on primary model failure

Since get_model_context_length() checks this value at resolution step 0 and returns it immediately, the new/fallback model inherits the old override instead of going through the full resolution chain (custom_providers per-model, endpoint metadata, models.dev, etc.).

Fix

Clear self._config_context_length = None in both code paths, before the runtime field swap. This allows get_model_context_length() to skip the stale step-0 override and properly resolve the context window for the newly selected model through the standard chain (step 0b: custom_providers per-model, then endpoint probe, models.dev, etc.).

Testing

1. Configure model.context_length: 1048576 in config.yaml for model A (1M context)
2. Start Hermes with model A — verify 1M context window
3. Switch to model B (e.g. 200K context) via /model — before fix: still shows 1M; after fix: correctly shows 200K
4. Switch back to model A — correctly shows 1M again
5. Trigger a fallback (e.g. rate-limit the primary model) — before fix: fallback model inherits primary's context window; after fix: fallback model resolves its own context window

Closes #21509