fix(capture): align malformed window marker error

[abrichr]

What changed

Align the fallback capture_window validation error with the existing
fail-closed regression contract:

• keep rejecting non-object window-capture metadata;
• preserve the exact capture_window field name for diagnosis; and
• consistently describe the condition as malformed window-capture metadata.

Why

PR #146 added both the fail-closed validation and its optional-capture
regression, but the fallback branch used a different phrase than the test
contract. With openadapt-capture installed, the behavior was correct and the
test failed only because the error message did not match.

Impact

No capability, gate, or execution behavior changes. Malformed metadata still
halts conversion rather than falling back to unsafe full-screen scaling.

Verification

• direct _window_capture_meta regression: malformed fallback metadata raises
  ValueError containing both malformed window-capture metadata and
  capture_window;
• Ruff lint and format checks pass;
• git diff --check passes.

The full optional-capture test module was not rerun on the local headless macOS
session because the separately installed capture checkout attempts a real
screen probe during import. Protected CI remains the definitive clean
environment gate.

[abrichr]

Post-v1.17.1 queue preparation (2026-07-19)

• Rebased exact head: 685fd67238cc7ee8f1be78710d2e8989a442df95
• Exact base: 9a5018d6f1538e8680804f4c77192e4959f57344 (published v1.17.1 release commit)
• Scope remains one file and two message lines: corrupt capture_window metadata still raises before conversion; no capture capability, refusal, or scaling behavior changed.
• Direct malformed-marker check, Ruff lint/format, release-consistency, and diff checks pass locally.
• The previous exact head passed every protected check. Fresh protected claims/static/package checks are green on this rebased head; full unit and browser jobs are still running.
• fix(capture): align malformed window marker error #154 and attended-action feat(attended): execute governed halt actions #152 have zero changed-path overlap. A synthetic git merge-tree of feat(attended): execute governed halt actions #152 over this exact head is conflict-free.

This remains draft and unmerged. Queue order is intentionally:

1. Confirm the v1.17.1 managed runtime is promoted in production.
2. Merge fix(capture): align malformed window marker error #154 and verify exact-main/release state.
3. Rebase feat(attended): execute governed halt actions #152 once on that exact result and run fresh protected CI.

No publication or production mutation is being performed from this PR yet.