fix(react-email): don't leak a file handle when a static file can't be read - #3812

Fixes #3808.

email dev serves files under static/ with serveStaticFile(). It opened the file with fs.open() and only closed the handle after a successful read, so when the read fails after the open succeeds (a request for a directory under /static on Linux or macOS gives EISDIR; a file another program has locked on Windows gives EBUSY) the FileHandle is never closed.

Node.js 24 only prints a DEP0137 warning for that. Node.js 25 made it an error, so the preview server exits the next time the garbage collector runs:

Error: A FileHandle object was closed during garbage collection. This used to be allowed with a deprecation warning but is now considered an error. Please close FileHandle objects explicitly. File descriptor: 3 (...\emails\static\logo.png)
  code: 'ERR_INVALID_STATE'

Changes:

• Read the file with fs.readFile(path) instead of opening a handle first, so nothing stays open when the read fails. The 404/500 handling is unchanged.
• Add a test that makes the read fail and checks no opened handle is left open. It fails without the fix.
• Add a patch changeset for react-email.

Reproduction on Windows with Node.js 26.12: with emails/static/logo.png locked by another process, calling serveStaticFile() and then forcing GC crashes the process before this change; after it, the request gets a 500 and the process keeps running.

I used Claude Code to write this change.

Summary by cubic

Fixes #3808 and a related leak in @react-email/ui. email dev served files under static/ by opening a FileHandle and only closing it after a successful read; when the read failed (a directory request on Linux/macOS gives EISDIR, a locked file on Windows gives EBUSY), the handle leaked. Node.js 24 prints a DEP0137 warning for that, but Node.js 25 turns it into an ERR_INVALID_STATE error that crashes the preview server at the next garbage collection. The same pattern leaked a handle while scanning the emails directory, and an unreadable file there failed the whole scan.

Changes:

• Reads static files with fs.readFile() instead of opening a handle first, so nothing stays open when the read fails; the 404/500 handling is unchanged, and requesting a directory under /static now returns a 404 instead of a 500.
• Scans the emails directory with fs.promises.stat() and reads by path, skipping unreadable files with a warning instead of failing the scan.
• Adds tests that make reads fail and verify no descriptor stays open; they fail without the fix.
• Adds patch changesets for react-email and @react-email/ui.

^{Written for commit 0b05a8b. Summary will update on new commits.}