misc: Snapshot the environment when a switchenv is entered

[MaxFreedomPollard]

switchenv.__exit__ restores the environment with os.environ.clear() followed by os.environ.update(self.previous), but self.previous is captured in switchenv.__init__ (devito/parameters.py), so it dates from construction rather than from entry. SwitchDecorator.__call__ wraps the decorated function in with self, so a switchenv used as a decorator is built once, when the decorator is applied, and entered on every call. The first call therefore rewinds the environment to how it looked at decoration time, deleting every variable set since. Same story for a switchenv object kept around and reused as a context manager.

On main:

import os
from devito import switchenv

@switchenv({'FOO': 'inside'})
def f():
    return os.environ['FOO']

os.environ['SET_LATER'] = 'keepme'
f()
print(os.environ.get('SET_LATER'))  # None, was 'keepme'

The sibling switchconfig already rebuilds self.previous at the top of __enter__. This does the same in switchenv, taking the snapshot before the device vars are popped so that a single use restores exactly what it restored before. Every existing call site builds the object inline in a with statement, where construction and entry are adjacent, so none of them change behaviour.

Verified on macOS with DEVITO_ARCH=clang. The new tests/test_tools.py::test_switchenv_reuse fails on unmodified main with KeyError: 'TEST_VAR_LATE' and passes with the fix. pytest tests/test_tools.py gives 32 passed and pytest tests/test_environment.py gives 1 passed. isort --check-only ., ruff check --preview, flake8 --builtins=ArgumentError . and typos are all clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.40%. Comparing base (33c3b12) to head (6c439c3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3019      +/-   ##
==========================================
- Coverage   83.77%   79.40%   -4.38%     
==========================================
  Files         257      257              
  Lines       55007    55025      +18     
  Branches     4708     4708              
==========================================
- Hits        46083    43692    -2391     
- Misses       8109    10467    +2358     
- Partials      815      866      +51     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	?
pytest-gpu-gcc-	?
pytest-gpu-icx-	?
pytest-gpu-nvc-nvidiaX	?

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[EdCaunt]

Needs a rebase, but then it's good to go

[MaxFreedomPollard]

Rebased on main.