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 40.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.00%. Comparing base (f09f8af) to head (176809a).
⚠️ Report is 859 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	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3227      +/-   ##
============================================
+ Coverage     56.12%   60.00%   +3.88%     
- Complexity      976     1430     +454     
============================================
  Files           119      170      +51     
  Lines         11743    15758    +4015     
  Branches       2251     2604     +353     
============================================
+ Hits           6591     9456    +2865     
- Misses         4012     4982     +970     
- Partials       1140     1320     +180     

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