fix(ci): support python 3.15 builds and make import profiler warning-only

[hebaalazzeh]

This PR resolves build failures on Python 3.15 and updates the import profiler checks to log warning messages instead of failing GHA checks, preventing any developer workflow blockages.

Key Changes

1. Python 3.15 Compatibility:
   • Exported PYO3_USE_ABI3_FORWARD_COMPATIBILITY=1 inside the temporary virtual environment block in run_single_test.sh. This ensures Rust-based transitive dependencies (like libcst used by bigframes) build successfully on Python 3.15.
2. Warning-Only Threshold Checks:
   • Increased the absolute failure threshold to 20000ms.
   • Updated both regression and absolute threshold checks to output strong WARNING messages instead of failing the check (the script will exit with code 0). This ensures the CI check status remains green while still providing performance regression visibility in the logs.

[gemini-code-assist]

Code Review

This pull request updates the import profiler and CI configuration to make import time regressions non-blocking. Specifically, it increases the failure threshold in ci/run_single_test.sh and modifies scripts/import_profiler/profiler.py to log warnings instead of failures and avoid setting a non-zero exit code. The reviewer suggests updating the warning messages in profiler.py to remove misleading references to "bypassing failure" since failures are no longer triggered.