Update docs for Python 3.15 support.

[junkmd]

1. Summary

This PR updates the project documentation to clarify Python 3.15 compatibility and the release status for enum-related issues.
It also adds a dedicated guide for the native C++ COM test server so the purpose, safety expectations, and CI requirements are documented explicitly.

2. Motivation & Background

Python 3.15 introduces changes in enum/IntFlag behavior that affected older comtypes releases.
The README now makes the compatibility status clear: versions up to 1.4.17 had issues with Python 3.15 enum behavior, while 1.5.0 is the release that supports Python 3.15.
This change is intended to reduce confusion for users and make the release boundary easier to understand.

Alongside that, the project adds a native COM test server README to document why a real native test double is needed, how it should be maintained, and what expectations exist for registration, unregistration, and CI verification.
This supports reliable cross-boundary COM testing without relying on ambient third-party components.

3. Key Changes

• README.md:
  • Updated the Python 3.15 availability note to clarify the enum compatibility issue and the fact that version 1.5.0 supports Python 3.15.
• source/CppTestSrv/README.md:
  • Added documentation for the native C++ COM test server, including its purpose, modification policies, memory safety expectations, deterministic validation rules, and CI/registration requirements.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (91408c2) to head (4925800).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #945   +/-   ##
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  Coverage   89.34%   89.34%           
=======================================
  Files         142      142           
  Lines       13856    13856           
=======================================
  Hits        12380    12380           
  Misses       1476     1476           

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[junkmd]

Once Python 3.15 is officially listed as "stable": true and "final" in the versions manifest (https://github.com/actions/python-versions/blob/main/versions-manifest.json), allowing it to be used in CI without allow-prereleases: true, we can release Version 1.5.0 of this package.