test: add error handling tests for SQL parser

[aviralgarg05]

Add unit tests for error handling in the SQL parser.

Tasks:

• Add tests for syntax errors in CREATE TABLE
• Add tests for invalid WHERE clauses
• Add tests for malformed INSERT statements
• Add tests for edge cases in SELECT parsing
• Ensure error messages are clear and helpful

Files to modify: nexum_core/src/sql/parser.rs

Good First Issue - Basic Rust knowledge helpful
Estimated Time: 2-3 hours

[noor05-creator]

Hi! I'd like to work on this issue.
I've been going through the parser code and noticed there's good coverage for successful parsing cases, but adding error handling tests would make the codebase more robust. I plan to add comprehensive tests for malformed SQL statements to ensure users get clear, actionable error messages.
My approach:

Test syntax errors (missing keywords, invalid tokens, etc.)
Test malformed INSERT statements (mismatched columns/values, invalid data types)
Test invalid WHERE clauses (unsupported operators, malformed expressions)
Test edge cases in SELECT (empty projections, invalid table names)
Verify that error messages are descriptive and helpful

I have experience with the codebase from working on the error message improvements (#89), so I'm familiar with the project structure and testing conventions.
Looking forward to contributing!

please assign this issue to me under OScG'26