Feat/agent thinking switch (#15446)

### What problem does this PR solve?

This PR adds an Agent LLM setting to control thinking mode for official
providers that expose a thinking switch.

Related to #12842.  
Closes #15445.

Some providers expose thinking controls through provider-specific
request fields, but Agent LLM settings did not have a unified option for
users to enable or disable thinking mode.

This PR adds a `Thinking` selector with:

- System default
- Enabled
- Disabled
<img width="452" height="278" alt="8566b0b4-0546-4c8a-913d-f9bbd38319f6"
src="https://github.com/user-attachments/assets/25b497f7-1ba0-4bfe-940d-6fe79287d6ab"
/>
<img width="471" height="971" alt="8a0a6bee-f45f-48d5-bd83-17af260de3db"
src="https://github.com/user-attachments/assets/41ad43c1-5087-48f1-bf37-f2ca14c2be2f"
/>
Initial support is limited to the verified official providers:

- Qwen / DashScope: `enable_thinking`
- Kimi / Moonshot: `thinking.type`
- GLM / ZHIPU-AI: `thinking.type`

For LiteLLM-based providers, provider-specific fields are forwarded
through `extra_body` before `drop_params` filtering so the request
parameters are preserved.



### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: jiashi <jiashi19@outlook.com>
Co-authored-by: Zhichang Yu <yuzhichang@gmail.com>

agent/component/llm.py

@@ -49,6 +49,7 @@ class LLMParam(ComponentParamBase):
         self.output_structure = None
         self.cite = True
         self.visual_files_var = None
+        self.thinking = ""
 
     def check(self):
         self.check_decimal_float(float(self.temperature), "[Agent] Temperature")
@@ -77,6 +78,8 @@ class LLMParam(ComponentParamBase):
             conf["presence_penalty"] = float(self.presence_penalty)
         if float(self.frequency_penalty) > 0 and get_attr("frequencyPenaltyEnabled"):
             conf["frequency_penalty"] = float(self.frequency_penalty)
+        if get_attr("thinking") in {"enabled", "disabled"}:
+            conf["thinking"] = get_attr("thinking")
         return conf