Capture this explicitly in the InterfaceAPI find callback (backport #1699)

[mergify]

🦟 Bug fix

Part of gazebosim/gz-cmake#580

Summary

[=] implicitly captured this, which is deprecated in C++20. The lambda only uses this->modelDir, so the capture is now explicit. No behavior change, and it is valid C++17, so it can land before any standard bump.

Before:

test/integration/interface_api.cc:151:9: warning: implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20 [-Wdeprecated]

Backport Policy

• This is safe to backport to the following versions:
  • Jetty
  • Ionic
  • Harmonic
  • Fortress
• This should not be backported
• I am not sure
• Other (fill in yourself)

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
• While waiting for a review on your PR, please help review another open pull request to support the maintainers
• Was GenAI used to generate this PR? If so, make sure to add "Assisted-by" to your commits. (See this policy for more info.)

Assisted-by: Claude Opus 5

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by and Generated-by messages.

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This is an automatic backport of pull request #1699 done by [Mergify](https://mergify.com).