fix: fix tool-search in response api

[hustxiayang]

Description

Currently, ai-gateway did not recognize tool_search_call and tool_search_output in responses input/output item unions. This caused two errors:

1. When parsing a response output item for a non-streaming request, ai-gateway can fail a valid upstream response with: unknown_type_field_value_'tool_search_call'_for_response_output_item_union
   This is because the response output union does not know that type, parsing hits the output-union default error:

   ai-gateway/internal/apischema/openai/openai.go

   Line 6452 in 6722cca

   func (r *ResponseOutputItemUnion) UnmarshalJSON(data []byte) error {

   . For streaming requests, this error is not visible because ai-gateway skips SSE event decode errors.

2. When parsing a later request that resends prior output items as input, ai-gateway would reject the request with:
   cannot unmarshal unknown input type: tool_search_call This is because response input union does not know that type:

   ai-gateway/internal/apischema/openai/openai.go

   Line 3984 in 6722cca

   func (r *ResponseInputItemUnionParam) UnmarshalJSON(data []byte) error {

Fix Add tool_search_call and tool_search_output to both unions.
Other Schema Items Added:

• additional_tools
• compaction_trigger
• computer_call_output
• custom_tool_call_output
• mcp_approval_response

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.42857% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.81%. Comparing base (e2c59d3) to head (d513123).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/apischema/openai/openai.go	71.42%	10 Missing and 10 partials ⚠️

❌ Your patch status has failed because the patch coverage (71.42%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2434      +/-   ##
==========================================
- Coverage   84.86%   84.81%   -0.05%     
==========================================
  Files         154      154              
  Lines       22419    22489      +70     
==========================================
+ Hits        19026    19075      +49     
- Misses       2237     2248      +11     
- Partials     1156     1166      +10     

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