fix(library): close sqlite and PDF temp handles so tests pass on Windows

[hudsonwa]

Summary

A fresh Windows checkout fails scripts/verify.sh at the pytest step (18 failures) because several tests and one production helper assumed POSIX semantics where an open file handle does not block another process from opening that path. On Windows, an open handle prevents deletion/opening, so:

• The sqlite-backed library tests used with sqlite3.connect(path) as db:, which commits on exit but does not close the connection. tempfile.TemporaryDirectory.cleanup() in tearDown therefore failed with PermissionError [WinError 32] (17 of the 18 failures, in tests/library/).
• _write_artifacts in quantmind/preprocess/format/pdf.py kept a NamedTemporaryFile handle open while handing source.name to the native liteparse parser, which then could not open the path (PDF error: file not found) — the 18th failure.

This PR fixes both root causes:

• Close the sqlite connection after each with sqlite3.connect(...) as db: helper block in the four library test files (db.close() immediately after the block). This preserves the block's commit-on-exit semantics while releasing the file handle so the temp dir can be cleaned up.
• For the PDF artifact helper, use delete=False, close the temp file handle before passing the path to the native parser, and unlink the file afterward in a finally.

These are test-infra / Windows-portability fixes only; no production behavior changes.

Related Issue

Closes #143

Verification

Performed on a Windows 11 host (Python 3.12.10) with the .[dev] environment, on a fresh shallow clone:

• Before (clean checkout, unmodified): test_pdf.py::test_golden_preserves_pages_blocks_coordinates_and_artifacts fails with PDF error: file not found; tests/library/test_local.py, test_structure.py, test_paper.py fail with PermissionError [WinError 32] (17 failures).
• After this patch:
  • pytest tests/library/test_local.py tests/library/test_structure.py tests/library/test_paper.py tests/library/test_example_bundle.py tests/preprocess/format/test_pdf.py -> 40 passed
  • Full suite: 425 passed, 1 failed where the single failure is the pre-existing, unrelated DiscoverPRNewswireTests::test_cache_bust_retries_listing_404 timing flake (microsecond cache-bust timestamp collision in quantmind/preprocess/pr_newswire.py); it fails identically on an unmodified master checkout and is not touched by this change.
  • ruff check on the changed files -> "All checks passed!"; ruff format --check -> all changed files formatted.

scripts/verify.sh could not be run end-to-end on the Windows box because its shell script is checked out with CRLF line endings (fails under Git Bash on Windows with $'\r': command not found); its individual steps were run and verified as green as noted above.

Checklist

• The title uses English Conventional Commit format: type(scope): summary.
• The related issue or design discussion is linked when applicable.
• bash scripts/verify.sh passes.
• Every applicable live-network component smoke test passes, or this PR states why none applies.
• Public behavior has focused tests, an example, and documentation where applicable.
• The PR is complete, small, and contains no unrelated changes.