[thekevinbot]

Fixes #1585

Content is read by re-opening the path the scan found. If a FIFO or a device replaced the file between the scan and the read, the read blocked forever (FIFO) or never reached an end (/dev/zero). Now the re-open is non-blocking and checks the opened handle is a regular file. Anything else gives NULL content, the same as a file deleted in that window.

Sibling: #1584 (oversize file to NULL) also touches read_text in vtab.rs. This PR leaves the size handling alone.

Tests (red first)

• packages/rust/tests/path_table_content_non_regular.rs (integration): the scan runs in one statement, b.txt is swapped, and the next statement reads its content. The swaps are a directory, a device symlink (/dev/null; \\.\NUL on Windows), and a FIFO on Unix.
• packages/rust/tests/path_table_content_non_regular_e2e.rs (CLI e2e): a worker function swaps d/b.txt partway through a single dirsql query.
• Red CI run (red-test commit): https://github.com/thekevinscott/dirsql/actions/runs/41924028186/job/126903600358. On Linux, path_table_content_non_regular fails with left: [("directory", Some(None)), ("device", Some(Some(""))), ("fifo", None)]: the device reads as "" and the FIFO read never returns. On Windows both new tests already pass, because opening a directory or a NUL symlink there gives no content.

E2E Verification

• Ran e2e suites locally for every affected SDK (the content-reading tests only; the fix is confined to the content re-open)
• Python SDK e2e: pass (17 passed)
• TypeScript SDK e2e: pass (16 passed)
• Rust core e2e (if applicable): pass (path_table_content_non_regular_e2e, path_table_content_prefetch)
• packages/python/e2e-attestations/<branch>.json receipt written if packages/python changed (just e2e-attest-python)
• packages/ts/e2e-attestations/<branch>.json receipt written if packages/ts changed (just e2e-attest-ts)
• Command(s) run: uv run maturin develop --release; cargo build --release -p dirsql --features cli; testing-conventions e2e attest 'uv run python -m pytest tests/e2e/fresh_declared_repros_test.py tests/e2e/query_contract_test.py tests/e2e/root_at_construction_test.py -x -q'; pnpm build; testing-conventions e2e attest 'pnpm exec vitest run --dir tests/e2e fresh-declared-repros query-contract query-params'; cargo test -p dirsql --features cli --test path_table_content_non_regular --test path_table_content_non_regular_e2e --test path_table_content_prefetch
• Result summary: all pass. Checked by hand with the release binary: a worker swaps d/b.txt partway through the query for a FIFO, a /dev/zero symlink, a /dev/null symlink or a directory. Each run returns content: null and exits 0. Before the fix, the FIFO hung and /dev/zero was OOM-killed.

Changelog / Migrations

• Changelog fragment added under <root>/<pkg>/changelog.d/ for each changed package (or: skip-changelog trailer on a commit with reason)
• Migration fragment with the same slug added under <root>/<pkg>/migrations.d/ for each changed package (every subsection but Summary _None._ when nothing breaks)