tests(openai-agents): Remove MagicMock from ModelResponse

[alexander-alderman-webb]

Description

Instances of MagicMock cannot be serialized, but model responses are serialized during the truncation proposed in #5049.

The model ResponseFunctionToolCall does not declare a function field: https://github.com/openai/openai-python/blob/main/src/openai/types/responses/response_function_tool_call.py

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.94%. Comparing base (2397b15) to head (a5c0591).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5053      +/-   ##
==========================================
+ Coverage   83.93%   83.94%   +0.01%     
==========================================
  Files         179      179              
  Lines       17887    17887              
  Branches     3179     3179              
==========================================
+ Hits        15013    15015       +2     
+ Misses       1906     1904       -2     
  Partials      968      968              

see 3 files with indirect coverage changes