fix: stabilize OAuth tests and Windows npm staging

[rekram1-node]

Summary

Keep the confirmed, limited CI fixes without expanding into Windows junction repair.

• Assert TypeError for an invalid OAuth redirect instead of Bun-specific error wording.
• Wait for the actual OpenAI OAuth method and configured agent permission rule before testing their behavior. Catalog reads no longer wait for plugin initialization.
• Create registry fixture tarballs using a relative archive path so GNU tar does not interpret a Windows drive letter as a remote host.
• Defer entrypoint resolution for successfully staged npm installs until after the staging directory is renamed. The pre-rename entrypoint was unused; avoiding it prevents Bun resolver handles from blocking the rename on Windows. Preserve metadata reads, revision comparison, requested subpaths, and the missing-node validation fallback.
• Assert that Git installs return an entrypoint under their published package directory.

The later Windows junction-repair change has been reverted. No test skips, mutation retries, permission-policy changes, dependency changes, or schema-comment changes.

Validation

Bun 1.4.0 on macOS, for the retained changes:

• Full core suite: 4,063 passed, 40 skipped.
• Full server suite: 52 passed, 3 skipped.
• Fetch suite repeated five times: 60 passed.
• MCP OAuth suite: 6 passed.
• Npm suite: 13 passed.
• Util suite: 37 passed.
• Core, server, and util typechecks passed.
• git diff --check passed. Changed test files pass Prettier; existing formatting differences in util/npm.ts were left untouched.

Remaining Failures

The retained changes passed the Windows server suite and resolved the npm EPERM rename and tar-path errors. Two Windows Git dependency tests still fail because directory links point at the old staging location. Those are intentionally deferred.

The last run of the retained code also reported a Linux module-resolution error in the unchanged tool-edit test; that test and the full core suite pass locally. This PR does not claim all CI checks are green.