fix(ci): disable IPO for test coverage builds

[IvanaGyro]

Summary

• Disable CMAKE_INTERPROCEDURAL_OPTIMIZATION when RUN_TESTS=ON.
• Preserve the existing IPO behavior for non-test builds.

Why

LTO can cause the pybind/ translation units to disappear from Codecov coverage attribution. Disabling IPO for coverage builds keeps source-level coverage data associated with the original files.

Validation

No local tests were run per request; CI should verify the change.

Fixes #1162

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.47%. Comparing base (a969a66) to head (4f51673).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1163      +/-   ##
==========================================
+ Coverage   67.92%   71.47%   +3.55%     
==========================================
  Files         208      208              
  Lines       22355    22355              
  Branches       72       72              
==========================================
+ Hits        15185    15979     +794     
+ Misses       7148     6354     -794     
  Partials       22       22              

Flag	Coverage Δ
cpp	71.60% <ø> (+3.61%)	⬆️
python	63.96% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
C++ backend	71.60% <ø> (+3.61%)	⬆️
Python bindings	∅ <ø> (∅)
Python package	63.96% <ø> (ø)
see 11 files with indirect coverage changes

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a969a66...4f51673. Read the comment docs.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.