test(sql): add quoted-identifier tests for DESCRIBE/DROP

[aviralgarg05]

Summary

Add parser tests for quoted identifiers in management statements.

Why

DESCRIBE and DROP TABLE accept cleaned identifiers; we should validate behavior for backticks, single quotes, and double quotes.

Tasks

• Extend tests in nexum_core/src/sql/parser.rs
• Add cases for:
  • DESCRIBE "users"
  • DROP TABLE users``
  • DROP TABLE IF EXISTS 'users'
• Assert parsed table name matches expected identifier

Acceptance Criteria

• New tests pass with cargo test --workspace -- --test-threads=1
• No parser behavior regressions

[coderabbitai]

🔗 Related PRs

#137 - feat: add transaction support (BEGIN/COMMIT/ROLLBACK) with WAL recovery [merged]

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