fix: Prevent native write when input is not Arrow format

[andygrove]

Rationale

Closes #2944
Closes #3215

Summary

• Adds planning-time check in CometExecRule to prevent native write when input data source is not a CometNativeExec (i.e., produces non-Arrow data like OnHeapColumnVector)
• Improves error message in Utils.scala to explain the issue and suggest workarounds
• Adds test to verify fallback behavior works correctly

Test plan

• Added test "native write falls back when scan produces non-Arrow data" in CometParquetWriterSuite
• Test verifies that when native_comet scan doesn't support complex types, the native write falls back to Spark instead of failing at runtime
• All existing tests pass

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 35.29412% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.05%. Comparing base (f09f8af) to head (0a5fec1).
⚠️ Report is 878 commits behind head on main.

Files with missing lines	Patch %	Lines
.../scala/org/apache/spark/sql/comet/util/Utils.scala	0.00%	8 Missing ⚠️
...n/scala/org/apache/comet/rules/CometExecRule.scala	57.14%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3227      +/-   ##
============================================
+ Coverage     56.12%   60.05%   +3.92%     
- Complexity      976     1442     +466     
============================================
  Files           119      172      +53     
  Lines         11743    15941    +4198     
  Branches       2251     2633     +382     
============================================
+ Hits           6591     9573    +2982     
- Misses         4012     5038    +1026     
- Partials       1140     1330     +190     

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[parthchandra]

Would it be more generic to do this in CometExecRule.convertToComet -

 if (op.children.nonEmpty && !op.children.forall(_.isInstanceOf[CometNativeExec])) {
    // For  operators like writes, require all children to be native
    if (requiresNativeChildren(handler)) {
      return None  // Fallback to Spark
    }
   ...
  }

[andygrove]

Yes, thanks. I updated this.

[comphead]

Great thanks @andygrove

[andygrove]

Thanks for the reviews @parthchandra and @comphead