refactor: refactor the datafusion planner

[ChunxuTang]

This PR refactors the datafusion planner into multiple files:

• mod.rs
• analysis.rs: query analysis phase
• builder.rs: datafusion plan building phase
• config_helpers.rs: shared utilities
• expression.rs: convert to datafusion Expr objs
• join_ops.rs: assemble relationship joins
• scan_ops.rs: constructs node/relationship scans
• test_fixtures.rs: shared test utilities

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.28645% with 210 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@d843b4b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ust/lance-graph/src/datafusion_planner/scan_ops.rs	82.44%	66 Missing ⚠️
rust/lance-graph/src/datafusion_planner/builder.rs	95.79%	53 Missing ⚠️
...t/lance-graph/src/datafusion_planner/expression.rs	93.20%	32 Missing ⚠️
...ust/lance-graph/src/datafusion_planner/join_ops.rs	92.91%	28 Missing ⚠️
rust/lance-graph/src/datafusion_planner/mod.rs	88.48%	19 Missing ⚠️
...ust/lance-graph/src/datafusion_planner/analysis.rs	97.66%	6 Missing ⚠️
...nce-graph/src/datafusion_planner/config_helpers.rs	96.40%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage        ?   81.68%           
=======================================
  Files           ?       23           
  Lines           ?     7846           
  Branches        ?        0           
=======================================
  Hits            ?     6409           
  Misses          ?     1437           
  Partials        ?        0           

Flag	Coverage Δ
rust-unittests	81.68% <93.28%> (?)

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