VReplication Atomic Copy Workflows: fix bugs around concurrent inserts

[rohit-nayak-ps]

Description

Setting --vreplication-parallel-insert-workers greater than 1, had not been tested earlier for atomic copies. When a user attempted to do it, it caused a panic in vttablet. There were two fixes needed here:

1. The response needed to be cloned before passing it to the concurrent workers. This had been done for regular workflows but was missed in the atomic copy case.
2. The copy state for the last table was deleted incorrectly before the queue was closed (and all concurrent workers had stopped inserting).

In addition the TestFKWorkflow e2e test has been changed to test with concurrent inserts.

Related Issue(s)

Fixes #17716

Checklist

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Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 67.95%. Comparing base (22cf8b6) to head (0d24a94).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
...ablet/tabletmanager/vreplication/vcopier_atomic.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17772      +/-   ##
==========================================
- Coverage   67.95%   67.95%   -0.01%     
==========================================
  Files        1585     1586       +1     
  Lines      255162   255207      +45     
==========================================
+ Hits       173408   173432      +24     
- Misses      81754    81775      +21     

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[shlomi-noach]

Great catch! Minor code beauty suggestion, at your discretion.