update grammar file for maintainability

[jrgemignani]

Consolidated duplicate code, added helper functions, and reviewed the grammar file for issues.

NOTE: I used an AI tool to review and cleanup the grammar file. I
have reviewed all of the work it did.

Improvements:

1. Added KEYWORD_STRDUP macro to eliminate hardcoded string lengths
2. Consolidated EXPLAIN statement handling into make_explain_stmt helper
3. Extracted WITH clause validation into validate_return_item_aliases helper
4. Created make_default_return_node helper for subquery return-less logic

Benefits:

• Reduced code duplication by ~150 lines
• Improved maintainability with helper functions
• Eliminated manual string length calculations (error-prone)

All 29 existing regression tests pass

modified: src/backend/parser/cypher_gram.y

[MuhammadTahaNaveed]

@jrgemignani I don't think there's a need for adding a separate test file just for these tests. These tests are already in abundance in other test files. Maybe in future we should have a separate one for grammar, but it should be a comprehensive test suite and should cover all constructs.

[jrgemignani]

@jrgemignani I don't think there's a need for adding a separate test file just for these tests. These tests are already in abundance in other test files. Maybe in future we should have a separate one for grammar, but it should be a comprehensive test suite and should cover all constructs.

removed.