fix(tests): make screenshot expectations platform-aware

[Audacity88]

Summary

• Base branch: master (all contributions)
• What changed and why:
  • Limits the screenshot command output-path assertion to platforms where ScreenshotTool::screenshot_command(...) returns a supported command.
  • Adds an explicit unsupported-platform regression test that expects screenshot_command(...) to return None.
  • Removes the Windows failure mode where this unit test unwrapped None even though screenshots are not currently supported there.
• Scope boundary: This PR only covers the screenshot test/platform expectation gap from [Bug]: 74 test failures on Windows — Unix-only test commands, path semantics, console encoding #7462. It does not add Windows screenshot support, change screenshot runtime behavior, or address the remaining updater, console-encoding, path-semantics, or CI-matrix work.
• Blast radius: Test-only change in zeroclaw-tools screenshot coverage. Runtime behavior is unchanged.
• Linked issue(s): Related [Bug]: 74 test failures on Windows — Unix-only test commands, path semantics, console encoding #7462. Related [Feature]: Run the test suite on Windows and macOS in CI, not just Linux #7461.
• Labels: bug, risk: high, size: XS, tool, tool:browser

Validation Evidence (required)

• Commands run and tail output:

cargo fmt -p zeroclaw-tools -- --check
result: passed

cargo test -p zeroclaw-tools screenshot --lib
running 7 tests
test screenshot::tests::screenshot_command_exists ... ok
test screenshot::tests::screenshot_command_contains_output_path ... ok
test screenshot::tests::screenshot_tool_name ... ok
test screenshot::tests::screenshot_tool_description ... ok
test screenshot::tests::screenshot_tool_spec ... ok
test screenshot::tests::screenshot_tool_schema ... ok
test screenshot::tests::screenshot_rejects_shell_injection_filename ... ok
test result: ok. 7 passed; 0 failed; 0 ignored; 0 measured; 1330 filtered out

cargo clippy -p zeroclaw-tools --lib -- -D warnings
result: passed

git diff --check
result: passed

• Beyond CI — what did you manually verify? Confirmed the diff is limited to crates/zeroclaw-tools/src/screenshot.rs and that the new unsupported-platform expectation matches the existing screenshot_command implementation.
• If any command was intentionally skipped, why: Workspace-wide clippy and nextest were not run locally for this test-only XS slice; the focused zeroclaw-tools test and clippy coverage exercise the touched module.

Security & Privacy Impact (required)

• New permissions, capabilities, or file system access scope? No
• New external network calls? No
• Secrets / tokens / credentials handling changed? No
• PII, real identities, or personal data in diff, tests, fixtures, or docs? No
• If any Yes, describe the risk and mitigation: N/A

Compatibility (required)

• Backward compatible? Yes
• Config / env / CLI surface changed? No
• If No or Yes to either: N/A

Rollback (required for risk: medium and risk: high)

• Fast rollback command/path: git revert <merge-sha>
• Feature flags or config toggles: None.
• Observable failure symptoms: Screenshot-focused unit tests fail on unsupported platforms, especially Windows, while runtime screenshot behavior remains unchanged.

Supersede Attribution (required only when Supersedes # is used)

• Superseded PRs + authors (#<pr> by @<author>, one per line): N/A
• Scope materially carried forward: N/A
• Co-authored-by trailers added in commit messages for incorporated contributors? No
• If No, why (inspiration-only, no direct code/design carry-over): No superseded PR or contributor-authored code is carried forward.

[singlerider]

I checked the test gating against the screenshot_command implementation.

🟢 What looks good: the cfg gates mirror the production branches exactly

screenshot_command returns Some only for target_os = "macos" and target_os = "linux", and None otherwise. The existing test now carries #[cfg(any(target_os = "macos", target_os = "linux"))] and the new test carries the exact negation, asserting None. The test coverage tracks the real platform branches with no gap and no overlap, so the suite stays meaningful on Windows and other targets instead of failing on an assumption that only holds on two platforms.

Test-only change, CI green, no outstanding reviews.