fix: use context manager for file read in pdf_renderer.py

[loganbail]

What I found

The _requires_xelatex helper in backend/app/services/pdf_renderer.py (line 55) opens a .tex file and reads the first 4096 bytes without a context manager:

content = open(main_tex_path, "r", encoding="utf-8", errors="ignore").read(4096)

The file handle is never explicitly closed. In CPython with refcounting GC it gets collected quickly, but under PyPy or if read() itself raises, the handle leaks. This is especially relevant because _requires_xelatex is called inside compile_latex_project — during a LaTeX compilation run, multiple files could be probed.

The fix

Wrapped the read in a with statement so the file handle is deterministically closed:

with open(main_tex_path, "r", encoding="utf-8", errors="ignore") as f:
    content = f.read(4096)

The existing encoding="utf-8" and errors="ignore" parameters are preserved — the only change is ensuring deterministic file handle cleanup.

Testing

Added tests/test_pr_07_pdf_renderer_file.py with five tests:

Test	What it checks
test_requires_xelatex_with_ctexart	Returns True for ctexart document class
test_requires_xelatex_with_standard_latex	Returns False for standard article class
test_requires_xelatex_with_fontspec	Returns True when fontspec package is used
test_requires_xelatex_missing_file	Returns False for nonexistent files (no crash)
test_no_unclosed_file_reads	Golden check: no open(...).read(...) patterns remain

All five pass.