[codex] Improve filesystem startup directory diagnostics

[tarunag10]

Summary

This improves filesystem server startup diagnostics for invalid allowed-directory arguments.

The current source already skips invalid directories and only exits when none of the specified directories are accessible, which matches the behavior requested in #4238. However, the stderr warnings for inaccessible paths were still generic and the hard-fail path did not list the invalid arguments or their concrete failure reasons. In host environments that hide or truncate child-process stderr, every extra bit of actionable startup output matters.

Root Cause / User Impact

A missing or invalid allowed directory can be difficult to diagnose from a host-level "transport closed" failure. The current startup validation says that a directory cannot be accessed, but it drops the underlying error message such as ENOENT, and when all directories are invalid it exits without enumerating the failed inputs.

Fix

• Record invalid allowed directories during startup validation with a concrete reason.
• Emit warnings in a consistent Skipping invalid allowed directory format that includes the directory and reason.
• When no requested directories are accessible, print a compact invalid-directory list before exiting.
• Tighten the startup validation test harness so child processes are terminated after the server emits its startup banner instead of relying only on a blind timeout.
• Extend startup tests to assert missing-directory stderr includes the path and ENOENT, and that all-invalid startup reports each failed directory.

Validation

• npm ci
• npm run build --workspace @modelcontextprotocol/server-filesystem
• npx vitest run src/filesystem/__tests__/startup-validation.test.ts --config src/filesystem/vitest.config.ts
• npm test --workspace @modelcontextprotocol/server-filesystem
• git diff --check

Fixes #4238.