feat(config): add when_thinking_disabled support for model configs (#1970)

* feat(config): add when_thinking_disabled support for model configs

Allow users to explicitly configure what parameters are sent to the
model when thinking is disabled, via a new `when_thinking_disabled`
field in model config. This mirrors the existing `when_thinking_enabled`
pattern and takes full precedence over the hardcoded disable behavior
when set. Backwards compatible — existing configs work unchanged.

Closes #1675

* fix(config): address copilot review — gate when_thinking_disabled independently

- Switch truthiness check to `is not None` so empty dict overrides work
- Restructure disable path so when_thinking_disabled is gated independently
  of has_thinking_settings, allowing it to work without when_thinking_enabled
- Update test to reflect new behavior

config.example.yaml

@@ -12,7 +12,7 @@
 # ============================================================================
 # Bump this number when the config schema changes.
 # Run `make config-upgrade` to merge new fields into your local config.yaml.
-config_version: 5
+config_version: 6
 
 # ============================================================================
 # Logging
@@ -50,6 +50,10 @@ models:
   #     extra_body:
   #       thinking:
   #         type: enabled
+  #   when_thinking_disabled:
+  #     extra_body:
+  #       thinking:
+  #         type: disabled
 
   # Example: OpenAI model
   # - name: gpt-4
@@ -88,6 +92,9 @@ models:
   #   when_thinking_enabled:
   #     thinking:
   #       type: enabled
+  #   when_thinking_disabled:
+  #     thinking:
+  #       type: disabled
 
   # Example: Google Gemini model (native SDK, no thinking support)
   # - name: gemini-2.5-pro
@@ -120,6 +127,10 @@ models:
   #     extra_body:
   #       thinking:
   #         type: enabled
+  #   when_thinking_disabled:
+  #     extra_body:
+  #       thinking:
+  #         type: disabled
 
   # Example: DeepSeek model (with thinking support)
   # - name: deepseek-v3
@@ -136,6 +147,10 @@ models:
   #     extra_body:
   #       thinking:
   #         type: enabled
+  #   when_thinking_disabled:
+  #     extra_body:
+  #       thinking:
+  #         type: disabled
 
   # Example: Kimi K2.5 model
   # - name: kimi-k2.5
@@ -153,6 +168,10 @@ models:
   #     extra_body:
   #       thinking:
   #         type: enabled
+  #   when_thinking_disabled:
+  #     extra_body:
+  #       thinking:
+  #         type: disabled
 
   # Example: Novita AI (OpenAI-compatible)
   # Novita provides an OpenAI-compatible API with competitive pricing
@@ -173,6 +192,10 @@ models:
   #     extra_body:
   #       thinking:
   #         type: enabled
+  #   when_thinking_disabled:
+  #     extra_body:
+  #       thinking:
+  #         type: disabled
 
   # Example: MiniMax (OpenAI-compatible) - International Edition
   # MiniMax provides high-performance models with 204K context window