Windows: subprocess environment propagation breaks with includeParentEnvironment=false

[tsavo-at-pieces]

Problem

Several subprocess environment tests fail on Windows:

• includes modifications to env
• includes scoped modifications to env
• includes values from the environment parameter
• environment parameter overrides env
• includeParentEnvironment: false ignores env
• includeParentEnvironment: false uses environment parameter

Reproduction

• Run: https://github.com/open-runtime/dart_cli_script/actions/runs/22376939954
• windows-x64 job: https://github.com/open-runtime/dart_cli_script/actions/runs/22376939954/job/64769258023
• windows-arm64 job: https://github.com/open-runtime/dart_cli_script/actions/runs/22376939954/job/64769258042

Observed failure modes

1. Case-sensitive key assertions fail (cli_script_test_* vs uppercase env keys on Windows).
2. includeParentEnvironment: false path can fail with Windows process/environment semantics.

Expected behavior

• Env assertions should work with Windows case-insensitive key semantics.
• includeParentEnvironment: false should behave predictably on Windows and not fail process start due to invalid/minimal env block behavior.

Proposed approach

1. Make env-related test assertions case-insensitive on Windows.
2. Revisit env construction when includeParentEnvironment: false and environment is null/empty.
3. Add dedicated Windows tests for:
   • explicit env override behavior
   • includeParentEnvironment=false with/without explicit env

Acceptance criteria

• All six env-related failures pass on windows-x64 and windows-arm64
• No regressions on Linux/macOS env tests
• Documented Windows-specific environment semantics where behavior is intentionally different