Fix rex parent_environ case sensitivity on Windows

[thc1006]

On Windows, environment variable keys are case-insensitive natively. os.environ preserves that contract through Python's os._Environ wrapper, but a plain dict copy of it (or any user-built dict) does not. Until now, ActionManager consumed whatever Mapping the caller passed as-is, so a package whose commands() referenced env.SomeVariable against a parent_environ that held the same variable under a different casing would raise:

Error in commands in package '...':
Referenced undefined environment variable: SomeVariable

Issue #2089 has a clean reproducer that follows exactly this shape: dict(os.environ) is passed to ResolvedContext.get_environ, and a package looks up env.SomeVariable while the dict only has SOMEVARIABLE.

The fix wraps a caller-supplied parent_environ in a small read-only case-insensitive proxy, gated on platform_.name == "windows". On Linux and macOS the assignment reduces to the existing identity passthrough, so non-Windows is byte-for-byte unchanged. The proxy is a 5-method Mapping private to rex.py; keys are upper-cased once at construction time so iteration matches what os.environ already yields on Windows, and last-write-wins on case collisions matches how os.environ itself collapses duplicates. The not isinstance(...) guard in the gate keeps wrapping idempotent.

The new test_parent_environ_case_insensitive_on_windows mirrors the issue's reproducer through _create_executor, asserting both direct parent_environ[...] lookup across casings and an end-to-end getenv("SOMEVARIABLE") / setenv("RESULT", ...) flow against a {"SomeVariable": ...} dict. It skips on non-Windows since the wrapping is a no-op there.

Verified locally: pytest src/rez/tests/test_rex.py is green (15 pass + 1 skip on Linux), full rez-selftest is green in a hermetic Python 3.11 container (Ran 259 tests in 24.997s, OK), flake8 is clean, and mypy | mypy-baseline filter reports no new violations above the pinned baseline.

Closes #2089

Reported by @nrusch. Independent diagnosis previously posted in #2093 (closed because the author could not satisfy the EasyCLA/DCO requirement, not for technical reasons).

[linux-foundation-easycla]

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• ✅ login: thc1006 / name: thc1006 (f3db9fb)

[thc1006]

Closing+reopening to retrigger Read the Docs (initial build failed during clone, 2s duration, generic clone error — fork branch is public and accessible).

[thc1006]

Status note for reviewers:

• The six core GitHub Actions workflows (tests, flake8, mypy, copyright, installation, benchmark) are queued in action_required state, awaiting maintainer approval per the first-time-contributor policy. Nothing to do on my side.
• The Read the Docs failure is git fetch origin pull/2098/head returning fatal: couldn't find remote ref pull/2098/head. The GitHub git API for this PR currently exposes only refs/pull/2098/merge; refs/pull/2098/head is missing from matching-refs/pull/2098, while other open PRs (e.g. Allow building with cmake on Windows #2090, Fix shell auto-completion #2091) have both refs as expected. An amend force-push was attempted to nudge GitHub into recreating the head ref but it has not reappeared. This appears to be a GitHub-side propagation glitch independent of the diff. Approving the workflows may incidentally fix it; if not, an RTD admin rebuild should clear it.
• DCO and EasyCLA are both green.

Local pre-push validation: pytest src/rez/tests/test_rex.py is green (15 pass + 1 Windows-only skip); full rez-selftest on a hermetic Python 3.11.14 container reports Ran 259 tests in 24.997s, OK (skipped=10); flake8 src/rez/rex.py src/rez/tests/test_rex.py is clean; mypy | mypy-baseline filter --ignore-categories=note --allow-unsynced reports no new violations above the pinned baseline.

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