feat: support COUNT(DISTINCT expr) in Cypher queries

[ChunxuTang]

This PR:

• Adds support for COUNT(DISTINCT x) aggregation in Cypher queries
• Introduces proper AST distinction between scalar and aggregate functions (similar to the handling in DataFusion)
• Validates COUNT(DISTINCT *) is rejected with a helpful error message

Fix #108

Example:

MATCH (p:Person)-[:workAt]->(o:Organisation)
RETURN COUNT(DISTINCT p.id) AS num_employees, o.id
ORDER BY num_employees DESC

[prrao87]

Exciting! Thanks for implementing 😁. This will unlock 10 queries in the LDBC benchmark suite!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.61224% with 23 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/lance-graph/src/semantic.rs	90.09%	11 Missing ⚠️
crates/lance-graph/src/parser.rs	87.93%	7 Missing ⚠️
...s/lance-graph/src/datafusion_planner/expression.rs	93.54%	4 Missing ⚠️
crates/lance-graph/src/simple_executor/expr.rs	75.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!