feat: add W014 case-without-else rule

[hellozzm]

Summary

Implements rule W014 which warns on CASE ... END expressions that lack an ELSE branch. Unmatched rows silently return NULL, which is often unintended.

Changes

• Added CaseWithoutElse rule class in sql_guard/rules/warnings.py
• Registered in sql_guard/rules/__init__.py
• Added fixture in tests/fixtures/warnings.sql
• Added tests (fires + does-not-fire) in tests/test_rules.py

Test plan

36 passed in 1.60s

Closes #4

Checklist

• ruff check . passes (N/A - no ruff installed in CI)
• pytest -q passes (36/36)
• Rule follows naming conventions (kebab-case, verb-oriented)
• Rule has docstring citing the SQL hazard

[github-actions]

Thanks for your first PR on sql-sop! Quick checks:

• pytest -q and ruff check . pass locally
• Every new rule has both a "fires on bad SQL" test AND a
  "does NOT fire on safe SQL" test
• Rule IDs (E001, W001, …) are public API — don't renumber
  or rename existing ones without the deprecation process in
  GOVERNANCE.md
• CHANGELOG.md [Unreleased] entry for any user-facing change

First-PR-wins on the linked issue applies for 7 days. The
maintainer will review within 14 days of your last response.

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[Pawansingh3889]

Thanks @hellozzm! Structure is right (multi-line check, follows the W019 shape), all 7 CI checks pass on your branch, and the test cases match the issue body cleanly.

That said, three things are off, with the first being a direct miss on an issue requirement.

Issue body required nested-CASE detection. Currently broken. Issue #4 specifically called out: "Test with nested CASE expressions - make sure outer CASE without ELSE still triggers when inner CASE has ELSE." The rule fails exactly that case:

SELECT CASE
   WHEN x THEN CASE WHEN y THEN 1 ELSE 2 END
   WHEN z THEN 3
END FROM t;