chore: Refactor CometExecRule handling of BroadcastHashJoin and fix fallback reporting

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

Fixing more legacy tech debt.

What changes are included in this PR?

• CometExecRule now delegates to CometBroadcastExchangeExec serde logic
• Fallback reasons are now reported for operators that have BroadcastExchangeExec children (such as BroadcastHashJoinExec)
• Removed CometExecRule.operator2Proto since it is no longer referenced
• Updated golden files now that fallback reasons are being reported

How are these changes tested?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.29%. Comparing base (f09f8af) to head (b7be893).
⚠️ Report is 741 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2856      +/-   ##
============================================
+ Coverage     56.12%   59.29%   +3.16%     
- Complexity      976     1350     +374     
============================================
  Files           119      167      +48     
  Lines         11743    15280    +3537     
  Branches       2251     2530     +279     
============================================
+ Hits           6591     9060    +2469     
- Misses         4012     4933     +921     
- Partials       1140     1287     +147     

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[comphead]

Thanks @andygrove good catch

[andygrove]

Thanks for the review @comphead. I'll go ahead and merge because I have some more changes to make that follow on from this one.