test(mcp): make OAuth browser launch injectable across platforms

[aheritier]

Problem

PR #3881 currently skips two POSIX PATH-shim browser-fixture tests on Windows because the runtime launches rundll32.

Goal

Introduce a test-only/injectable browser opener seam so browser-fixture tests are host-independent while preserving production behavior.

Acceptance criteria

• Windows executes the affected browser-fixture tests deterministically without relying on POSIX PATH shims.
• POSIX continues to execute the browser-fixture tests deterministically.
• Tests use a local injectable/test-only browser opener seam.
• Production browser-launch behavior remains unchanged.