fix: Avoid duplicate columns in shared variables

[ChunxuTang]

This PR adds support for reusing variables across multiple patterns in a Cypher query without introducing duplicate column errors. It implements logic in the DataFusion planner to detect when a target variable in a pattern is already bound in the schema, and applies a filter constraint (e.g., prev.id = next.id) instead of performing a redundant join.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.00000% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...tes/lance-graph/src/datafusion_planner/join_ops.rs	84.09%	14 Missing ⚠️
crates/lance-graph/src/logical_plan.rs	93.54%	4 Missing ⚠️

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