fix: Add INSERT statements to DAG planning

[ferenc-csaky]

Key Changes

• Model Flink SQL INSERT statements as DAG export nodes with source lineage.
• Preserve the original RichSqlInsert for execution, retaining target columns and modifiers.
• Move INSERT enqueueing into DAGPlanner for deterministic statement-set ordering.
• Add mutation-table INSERT regression coverage and update affected snapshots.

Fixes #1921

[codecov]

Codecov Report

❌ Patch coverage is 0% with 52 lines in your changes missing coverage. Please review.
✅ Project coverage is 18.74%. Comparing base (ea2cf01) to head (55a9e02).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...in/java/com/datasqrl/planner/SqlScriptPlanner.java	0.00%	22 Missing ⚠️
.../com/datasqrl/planner/Sqrl2FlinkSQLTranslator.java	0.00%	18 Missing ⚠️
...main/java/com/datasqrl/planner/dag/DAGPlanner.java	0.00%	8 Missing ⚠️
...ava/com/datasqrl/planner/dag/nodes/ExportNode.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2278      +/-   ##
============================================
- Coverage     18.78%   18.74%   -0.05%     
  Complexity     1223     1223              
============================================
  Files           630      630              
  Lines         18409    18455      +46     
  Branches       2250     2251       +1     
============================================
  Hits           3459     3459              
- Misses        14618    14664      +46     
  Partials        332      332              

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