fix(run_agent): use aux provider for compression context length lookup

Each auxiliary model must be resolved with its own provider so that
provider-specific paths (e.g. Bedrock static table, OpenRouter API)
are invoked for the correct client, not inherited from the main model.

When the main model is Bedrock, passing self.provider unconditionally
to get_model_context_length() for the aux model caused the Bedrock
static table hard-intercept (step 1b) to fire for non-Bedrock models,
returning BEDROCK_DEFAULT_CONTEXT_LENGTH=128K instead of the model's
real context window — triggering a false compression warning every session.

Fix: pass _aux_cfg_provider when explicitly set, falling back to
self.provider only when the aux provider is unset or "auto".

Closes #12977
Related: #13807, #17460

run_agent.py

@@ -2667,7 +2667,10 @@ class AIAgent:
                 base_url=aux_base_url,
                 api_key=aux_api_key,
                 config_context_length=getattr(self, "_aux_compression_context_length_config", None),
-                provider=getattr(self, "provider", ""),
+                # Each model must be resolved with its own provider so that
+                # provider-specific paths (e.g. Bedrock static table, OpenRouter API)
+                # are invoked for the correct client, not inherited from the main model.
+                provider=(_aux_cfg_provider if _aux_cfg_provider and _aux_cfg_provider != "auto" else getattr(self, "provider", "")),
             )
 
             # Hard floor: the auxiliary compression model must have at least