Fix abi3t config handling before Python 3.15

[cyphercodes]

Fixes #6176.

Summary

• Reuse the same stable-ABI version selection for PYO3_CONFIG_FILE configs as for live interpreter configs.
• Keep free-threaded Python versions before 3.15 version-specific, even when abi3t-py315 is enabled.
• Add regression coverage for the abi3 / abi3t feature priority around Python 3.14t and 3.15.

Tests

• cargo test -p pyo3-build-config
• PYO3_CONFIG_FILE=/tmp/oss-pr-pipeline/pyo3-314t-config.txt cargo check -p pyo3-ffi --features abi3-py310,abi3t-py315
• cargo fmt --check
• git diff --check

[codspeed-hq]

Merging this PR will not alter performance

✅ 140 untouched benchmarks

---

_{Comparing cyphercodes:fix-config-abi3t-free-threaded-314 (7474d13) with main (e2f8fb9)}

[cyphercodes]

Added focused coverage cases for the stable ABI target helper so the abi3t/no-abi3t and non-CPython paths are exercised.

Verification:

• cargo test -p pyo3-build-config stable_abi_version_for_target_matches_abi_support --lib
• cargo fmt --check
• git diff --check

[davidhewitt]

Respectfully, this PR (and your GitHub activity) looks strongly like automated contribution with no discussion with the maintainers about the proposed solution. While we have not formally merged our AI policy yet (#6141), this PR goes against the spirit of what we plan to ask from that policy. I will close this.