fix(toolchains): correctly register musl/freethreaded toolchains for workspace

[rickeylev]

The musl/freethreaded runtimes weren't being activated when the flags were set. This was
because the toolchains weren't having target_settings set, which means extra settings,
such as musl/freethreaded-ness were ignored when matching. The net result is the regular
toolchain, because it's registered earlier, would always match earlier.

To fix, set the target_settings in the toolchain() call. This matches the bzlmod behavior.

Also update the toolchain resolution tests to verify resolution.

Fixes #3262

[gemini-code-assist]

Summary of Changes

Hello @rickeylev, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical issue where musl and freethreaded Python runtimes were not being properly activated in Bazel WORKSPACE builds. The core problem stemmed from missing target_settings in toolchain registrations, which led to incorrect toolchain matching. The fix involves explicitly setting these target_settings to align with bzlmod behavior, ensuring that specific runtime variants are correctly identified and used. Additionally, new robust tests have been introduced to validate toolchain resolution for these variants, alongside a refactoring of test platform definitions for better maintainability.

Highlights

• Toolchain Resolution Fix: Correctly registers musl and freethreaded Python toolchains for WORKSPACE builds by setting target_settings in the toolchain() calls, which was previously missing.
• Improved Toolchain Matching: Ensures that when --py_linux_libc=musl or --py_freethreaded=yes flags are used, the appropriate toolchains are activated, preventing the default toolchain from being incorrectly matched.
• New Resolution Tests: Adds comprehensive toolchain resolution tests to verify the correct activation of musl and freethreaded variants across various platforms and Python versions.
• Test Infrastructure Refactoring: Consolidates platform definitions into a new dedicated tests/support/platforms directory, improving organization and consistency for test configurations.

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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue where musl and freethreaded toolchains were not being properly activated in WORKSPACE builds by ensuring target_settings are passed during toolchain registration. This change aligns the WORKSPACE behavior with the bzlmod implementation.

A significant portion of this PR involves refactoring the test infrastructure to centralize platform definitions, which greatly improves maintainability. Additionally, a comprehensive new test suite has been added to verify correct toolchain resolution across various configurations, which is an excellent addition to ensure the fix is robust.

I've found one critical issue in the new test implementation that would likely cause it to fail and have provided a suggestion for a fix. Overall, this is a high-quality contribution.