perf: Improve BroadcastExchangeExec conversion

[wForget]

Which issue does this PR close?

Closes #2416.

Rationale for this change

Currently, the BroadcastExchangeExec parent applies CometExecRule again after conversion, which will traverse all its children again.

datafusion-comet/spark/src/main/scala/org/apache/comet/rules/CometExecRule.scala

Line 449 in 9cac1af

val newPlan = apply(plan.withNewChildren(newChildren))

What changes are included in this PR?

How are these changes tested?

Existing CI

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.82%. Comparing base (f09f8af) to head (0222877).
⚠️ Report is 516 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/scala/org/apache/comet/rules/CometExecRule.scala	50.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2417      +/-   ##
============================================
+ Coverage     56.12%   57.82%   +1.69%     
- Complexity      976     1400     +424     
============================================
  Files           119      147      +28     
  Lines         11743    13469    +1726     
  Branches       2251     2341      +90     
============================================
+ Hits           6591     7788    +1197     
- Misses         4012     4448     +436     
- Partials       1140     1233      +93     

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[andygrove]

Thanks @wForget