Add context manager protocol for .NET IDisposable types

[Martin-Molinero]

Cherry-pick from upstream pythonnet/pythonnet: lets .NET IDisposable objects be used as Python context managers (with statement), calling Dispose() on exit.

• Upstream PR: Add context manager protocol for .NET IDisposable types pythonnet/pythonnet#2568 ("Add context manager protocol for .NET IDisposable types")
• Upstream commit: pythonnet/pythonnet@25e0ccf3 (by Denis Akhiyarov)
• Cherry-picked with -x (original authorship preserved).

Conflict note: only docs conflicted (CHANGELOG.md, doc/source/python.rst) — resolved to keep the upstream additions; no code conflicts. Builds clean; adds tests/test_disposable.py. Fits this fork's pattern of adding Pythonic protocols (__bool__, container methods, len).

🤖 Generated with Claude Code

[Martin-Molinero]

Closing — this can't pass on this fork as-is.

The upstream implementation (pythonnet#2568) adds __enter__/__exit__ via the CollectionMixinsProvider (ContextManagerMixin in Mixins/collections.py). This fork deliberately disabled CollectionMixinsProvider (see the // see https://github.com/pythonnet/pythonnet/issues/1785 comment in InteropConfiguration.cs), so the mixin is never injected and the new tests fail with AttributeError: __enter__.

Making this work would require re-enabling the mixins provider, which is a deliberate fork decision — out of scope for a cherry-pick. If you want IDisposable-as-context-manager, it'd need either re-enabling mixins (revisiting pythonnet#1785) or a C#-side implementation that doesn't depend on the mixin mechanism. Closing; happy to revisit if you decide to re-enable mixins.