fix: avoid os.environ mutation for color control

[frenck]

Description

syrupy signaled "disable color" by temporarily mutating os.environ (through the env_context helper). Mutating the environment calls glibc setenv, which is not thread safe and races with getenv called from native code in other threads. During the pytest_assertrepr_compare and diff_snapshots paths this could segfault unrelated code, for example a database driver running in a pytest-django live_server request handler, as reported in #955.

This replaces the environment mutation with a contextvars.ContextVar. terminal._is_color_disabled() now consults that ContextVar in addition to the external NO_COLOR and ANSI_COLORS_DISABLED variables, which are still read (reads are safe, only writes race). Both call sites, __terminal_color (when --snapshot-no-colors is set) and SnapshotReporter.diff_snapshots, use the new disable_color() context manager, so no os.environ writes happen anywhere in the path.

The now-unused env_context helper is removed from syrupy.utils. It was undocumented and not part of __all__, and was the source of the race. Its only remaining user was the osenv test fixture, which now defines the helper locally in conftest.py (setting a real environment variable inside a single threaded test is fine).

#956 narrowed when the mutation happened; this removes it entirely.

Related Issues

• Closes pytest_assertrepr_compare unnecessarily calls os.environ.update every time - increased segfault risk due to getenv not thread safe #955, intermittent segfaults caused by syrupy mutating os.environ from the assertion hook while other threads read the environment.

Checklist

• This PR has sufficient documentation.
• This PR has sufficient test coverage.
• This PR title satisfies semantic convention.

Additional Comments

New tests in tests/syrupy/test_terminal.py cover the toggle behavior, that the external NO_COLOR variable still disables color, and crucially that disable_color() does not touch os.environ. Behavior for users is unchanged: --snapshot-no-colors, external NO_COLOR/ANSI_COLORS_DISABLED, and the diff color suppression all work as before.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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##             main    #1125   +/-   ##
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  Coverage   97.43%   97.43%           
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  Files          24       24           
  Lines        1754     1754           
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  Hits         1709     1709           
  Misses         45       45           

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