fix(fallback): forward custom_providers to fallback model context-length detection

The same root cause as the auxiliary compression fix (commit 7becb19):
get_model_context_length() is called without custom_providers, so per-model
context_length overrides are silently skipped.  The fallback activation path
(_try_activate_fallback) had the same missing parameter.

When the agent switches to a fallback provider, the fallback model would use
the models.dev value (e.g. 204800 for NVIDIA NIM minimax-m2.7) instead of
the user-configured one in custom_providers (e.g. 196608) — a subtle
discrepancy that could cause the fallback model to run with an incorrect
context window, leading to truncated messages or failed API requests when
the model does not support the detected length.

Fix: pass self._custom_providers to get_model_context_length() so the
fallback path sees the same per-model overrides as the main model path.

run_agent.py

@@ -9187,6 +9187,7 @@ class AIAgent:
                     self.model, base_url=self.base_url,
                     api_key=self.api_key, provider=self.provider,
                     config_context_length=getattr(self, "_config_context_length", None),
+                    custom_providers=self._custom_providers,
                 )
                 self.context_compressor.update_model(
                     model=self.model,