Windows: make stderr and outputBytes assertions robust to locale and CRLF

[tsavo-at-pieces]

Problem

Two Windows failures are caused by platform-native output differences:

1. sub_process_test.dart: an error while spawning is printed to stderr
2. sub_process_test.dart: outputBytes returns the script's output as bytes

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

Root cause

• Spawn error text is OS/locale-dependent (and can differ in punctuation).
• print() output bytes are platform-native (\r\n on Windows vs \n on Unix).

Expected behavior

Tests should remain strict enough to validate behavior without requiring locale-specific exact strings or Unix-only line endings.

Proposed approach

1. Replace exact ProcessException string match with robust matcher (e.g. startsWith('ProcessException:') + stable prefix checks).
2. Make output bytes expectation platform-aware (Platform.lineTerminator) or normalize line endings in test assertions.
3. Add a short contract note in docs/comments that outputBytes returns raw platform-native bytes.

Acceptance criteria

• Both tests pass on windows-x64 and windows-arm64
• Both tests remain meaningful and green on Linux/macOS
• No weakening of failure signal (tests still verify correct behavior)