[Fetch Migration] Add track_total_hits parameter to doc-count operation

[kartg]

Description

The track_total_hits parameter forces an accurate doc-count even when the hit count for _search is above the default 10K limit. This resolves a bug where the total doc count was not being computed properly. Unit tests have also been updated to match the new path.

• Category: Bug fix

Testing

Tested emperically by performing a Fetch Migration between clusters, including an index with nested documents.
Unit tests:

$ python -m coverage report --omit "*/tests/*"
Name                           Stmts   Miss  Cover
--------------------------------------------------
endpoint_info.py                  24      0   100%
endpoint_utils.py                106      1    99%
fetch_orchestrator.py             72      0   100%
fetch_orchestrator_params.py      22      0   100%
index_diff.py                     20      0   100%
index_doc_count.py                 5      0   100%
index_operations.py               76      0   100%
metadata_migration.py             65      0   100%
metadata_migration_params.py       7      0   100%
metadata_migration_result.py       5      0   100%
migration_monitor.py              90      0   100%
migration_monitor_params.py        6      0   100%
progress_metrics.py               91      0   100%
utils.py                          13      0   100%
--------------------------------------------------
TOTAL                            602      1    99%

Check List

• New functionality includes testing
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (17d1555) 73.55% compared to head (70b630f) 73.67%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #474      +/-   ##
============================================
+ Coverage     73.55%   73.67%   +0.12%     
+ Complexity     1182     1180       -2     
============================================
  Files           124      124              
  Lines          4886     4886              
  Branches        439      439              
============================================
+ Hits           3594     3600       +6     
+ Misses          998      992       -6     
  Partials        294      294              

Flag	Coverage Δ
unittests	73.67% <ø> (+0.12%)	⬆️

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