feat: implement case-insensitive support

[ChunxuTang]

Core Feature:

• Implements case-insensitive support for all Cypher identifiers (labels, properties, variables, relationships, aliases)
• Fixes Issue Identifier case normalization causes failures (aliases + properties) #105 (camelCase properties not found)
• Aligns with SQL-like behavior

Implementation Details:

• Key normalization strategy: store as lowercase for O(1) lookups
• Modified GraphConfig, SourceCatalog, and SemanticAnalyzer to normalize keys
• Centralized column qualification in qualify_column() helper
• Updated all planners and executors to use normalized identifiers

Note that there's also a TODO about the GraphConfig API.

[ChunxuTang]

The CI test query::tests::executes_against_directory_namespace was failing because of a key mismatch in the namespace handling code.

Lance datasets were created with mixed-case filenames (e.g., Person.lance, FRIEND_OF.lance), but it stored providers in the map using the original case name. Fix the query.rs to use the lowercase names.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.26829% with 44 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/lance-graph/src/config.rs	87.95%	10 Missing ⚠️
crates/lance-graph/src/query.rs	80.00%	10 Missing ⚠️
...tes/lance-graph/src/datafusion_planner/scan_ops.rs	74.19%	8 Missing ⚠️
...aph/src/datafusion_planner/builder/join_builder.rs	50.00%	4 Missing ⚠️
...s/lance-graph/src/simple_executor/path_executor.rs	77.77%	4 Missing ⚠️
crates/lance-graph/src/case_insensitive.rs	98.17%	3 Missing ⚠️
...tes/lance-graph/src/datafusion_planner/join_ops.rs	86.66%	2 Missing ⚠️
crates/lance-graph/src/semantic.rs	88.23%	2 Missing ⚠️
...s/lance-graph/src/datafusion_planner/expression.rs	75.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[beinan]

lgtm, just a minor comments

[beinan]

If the input RecordBatch has two columns that differ only by case (e.g., fullName and Fullname), normalize_schema will produce a schema with duplicate column names, am I right?

[ChunxuTang]

That's a good point! Yes, the schema will have duplicate column names for the scenario you mentioned. But the issue will be later caught by the DataFusion table registration, so I don't think we need to validate again here.

I ran a test locally to verify that lance-graph throws an exception (from DataFusion) "PlanError: Schema error: Schema contains duplicate qualified field name person.fullname".