fix(ai): update OpenAI Responses API parameter name

OpenAI changed the parameter name in their Responses API from
`max_completion_tokens` to `max_output_tokens`. This caused requests
to fail with:

"Unsupported parameter: 'max_completion_tokens'. In the Responses API,
this parameter has moved to 'max_output_tokens'."

Root Cause:
- openai_llm_provider.rs:360 - Used rename="max_completion_tokens"
- openai_compatible_provider.rs:484 - Field name was max_completion_tokens
  (which becomes the JSON key without explicit rename)

Fix:
- Changed serde rename to "max_output_tokens" in both files
- Updated test to verify new parameter name
- Kept internal Rust field name as max_completion_tokens for clarity

Impact:
- OpenAI Responses API calls now use correct parameter
- Compatible with current OpenAI API specification
- No breaking changes to internal code (field name unchanged)

Verified:
- MCP server builds successfully
- Serialization uses correct JSON key "max_output_tokens"

Reference:
https://platform.openai.com/docs/api-reference/responses/create

Author: jruokola

crates/codegraph-ai/src/openai_compatible_provider.rs

@@ -480,7 +480,10 @@ struct ResponsesAPIRequest {
     input: String,
     #[serde(skip_serializing_if = "Option::is_none")]
     instructions: Option<String>,
-    #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(
+        rename = "max_output_tokens",
+        skip_serializing_if = "Option::is_none"
+    )]
     max_completion_tokens: Option<usize>,
     #[serde(skip_serializing_if = "Option::is_none")]
     reasoning_effort: Option<String>,

crates/codegraph-ai/src/openai_llm_provider.rs

@@ -357,7 +357,7 @@ struct OpenAIRequest {
     #[serde(skip_serializing_if = "Option::is_none")]
     instructions: Option<String>,
     #[serde(
-        rename = "max_completion_tokens",
+        rename = "max_output_tokens",
         skip_serializing_if = "Option::is_none"
     )]
     max_completion_tokens: Option<usize>,
@@ -461,7 +461,7 @@ mod tests {
         };
 
         let json = serde_json::to_string(&request).unwrap();
-        assert!(json.contains("\"max_completion_tokens\""));
+        assert!(json.contains("\"max_output_tokens\""));
         assert!(!json.contains("\"max_completion_token\""));
     }
 }