test: make the Python suite green on Windows

[TimelordUK]

Local runs on Windows had a standing 7-failure baseline that was green in CI. It was recorded as one problem — the missing .exe suffix — but probing each failure showed three unrelated causes, only one of which was the suffix.

test_quoted_columns (5 failures) — the .exe suffix, and the only genuine instance. Note this bites solely because the file guards with exists(); the ~35 other files that hardcode a suffixless path work anyway, because Windows CreateProcess appends .exe itself. Fixed with the same idiom test_in_between_operators.py and tests/comparison/engines.py already carry.

test_unnest — same omission, worse symptom, and not previously noticed because nothing failed: the release exists() check fails, the file falls through to its target/debug fallback, and CreateProcess then resolves that to sql-cli.exe. The tests passed while exercising the DEBUG binary.

test_sql_comments (1) — not the suffix at all. NamedTemporaryFile(mode='w') writes in the platform's preferred encoding, cp1252 here, so the tests carrying scientific notation (a0, pi, epsilon0, hbar) raised UnicodeEncodeError before the CLI was invoked. Fixed in the test with encoding='utf-8' rather than by requiring PYTHONUTF8=1 in the runner.

test_web_cte_advanced (1) — also not the suffix. --query-plan prints the AST and then executes the query anyway, so the test attempts a fetch it believes it is avoiding. A refused localhost connection is instant on Linux and ~2.4s on Windows, against a timeout=2. Timeout raised to 15s and the real cause recorded in place: --query-plan should not execute.

Local suite now 534 passed, 0 failed, with no env-var prefix needed.

Claude-Session: https://claude.ai/code/session_01JBEUeckCwmWXoWTpQUTDqP