test(server): add regression test for /schema upload size limit

[ax1s-x1zz]

Summary

The server's /schema upload path enforces MAX_UPLOAD_BYTES (50 MiB) and rejects oversized files with 413 PAYLOAD_TOO_LARGE (xazz-server/src/main.rs ~line 501), but this behavior has no test. A regression could silently allow unbounded uploads (disk DoS).

Context

• Constant: MAX_UPLOAD_BYTES in xazz-server/src/main.rs.
• Handler: handle_schema reads the multipart field, checks data.len() > MAX_UPLOAD_BYTES, and returns (StatusCode::PAYLOAD_TOO_LARGE, ...).
• Existing tests in #[cfg(test)] mod tests call handlers directly (see handle_execute tests) — no HTTP server is needed.
• The axum Multipart extractor is the hard part: you can build a Multipart from a reqwest::multipart-style body, or test the size check by extracting the reusable validation into a helper function that takes &[u8] and asserting on that helper directly.

Task

Add a test that proves an upload larger than MAX_UPLOAD_BYTES is rejected with 413. Two acceptable approaches:

1. Preferred: extract the size-check into a small pure helper (e.g. fn validate_upload_size(len: usize) -> Result<(), StatusCode>) used by handle_schema, and unit-test it with MAX_UPLOAD_BYTES and MAX_UPLOAD_BYTES + 1.
2. Or build a real multipart body and invoke handle_schema with it.

Definition of Done

• A test named something like oversized_upload_is_rejected passes.
• Boundary covered: == MAX_UPLOAD_BYTES allowed, +1 rejected.
• cargo test -p xazz-server passes; cargo fmt --all -- --check clean.

Notes

• This is the server crate; it links tokio/axum, so builds are a bit heavier than xazz-compiler, but cargo test -p xazz-server is the scope.
• User-facing error strings in the handler are English; if you add any, keep them English.