refactor: refactor the builder into multiple files

[ChunxuTang]

This PR refactor the large datafusion_planner/builder.rs (~2400 lines) into multiple files under a new builder folder:

• basic_ops: Basic operations (filter, project, sort, limit, offset, distinct)
• expand_ops: Graph traversal operations (expand, variable-length expand)
• aggregate_ops: Aggregation and grouping operations
• join_builder: Join inference and building
• helpers: Utility functions

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.18203% with 134 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@9959b3a). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...aph/src/datafusion_planner/builder/join_builder.rs	83.70%	65 Missing ⚠️
...graph/src/datafusion_planner/builder/expand_ops.rs	93.29%	37 Missing ⚠️
...ce-graph/src/datafusion_planner/builder/helpers.rs	85.71%	22 Missing ⚠️
...-graph/src/datafusion_planner/builder/basic_ops.rs	98.49%	7 Missing ⚠️
...ph/src/datafusion_planner/builder/aggregate_ops.rs	96.42%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #45   +/-   ##
=======================================
  Coverage        ?   82.03%           
=======================================
  Files           ?       28           
  Lines           ?     8482           
  Branches        ?        0           
=======================================
  Hits            ?     6958           
  Misses          ?     1524           
  Partials        ?        0           

Flag	Coverage Δ
rust-unittests	82.03% <92.18%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.