perf(curation): avoid redundant wrapper parse

[harshitethic]

Summary

• remove the second wrapper parse from reject_non_single_select
• keep the single-statement SELECT-type check plus the explicit PRAGMA/DESCRIBE/SHOW/SUMMARIZE refusal
• preserve valid FROM-first, parenthesized SELECT, VALUES, trailing-semicolon, and trailing-comment behavior
• add regression coverage proving a valid curation query is parsed once instead of twice

Verification

• git diff --check
• uv run ruff check src/hflow/curation.py tests/test_catalog_curation.py
• uv run pytest -q tests/test_catalog_curation.py -k reject_non_single_select

Fixes #481

[github-actions]

👋 Hi @harshitethic — thanks for the contribution! To keep starter issues available
for other contributors and give every pull request a real review, we accept
1 open pull request per contributor at a time.

You already have #531 open, so this one is being closed automatically.
Once your open pull request is merged or closed, feel free to reopen this one —
no work is lost.