Removes pauseless FSM scheme

[noob-se7en]

Problem
Server and controller can disagree on the segment commit protocol. The server decides whether to use pauseless or normal commit based on the pauselessConsumptionEnabled flag in StreamIngestionConfig.
But the controller's createFsm() first checks for a segment.completion.fsm.scheme override in the stream config — if someone sets this to "pauseless" without enabling pauselessConsumptionEnabled, the controller creates a PauselessSegmentCompletionFSM while the server does a normal commit.

Solution
Remove the segment.completion.fsm.scheme stream config override from SegmentCompletionManager.createFsm(). The controller now always derives the FSM scheme from PauselessConsumptionUtils.isPauselessEnabled() — the same source of truth the server uses. This eliminates any possibility of mismatch.

Backward Compatibility

Config	pauselessEnabled	Before (buggy)	After (fixed)
scheme=pauseless	true	Pauseless FSM	Pauseless FSM (no change)
scheme=pauseless	false	Pauseless FSM ❌	Normal FSM ✅
scheme=default	true	Normal FSM ❌	Pauseless FSM ✅
scheme=default	false	Normal FSM	Normal FSM (no change)
not set	true	Pauseless FSM	Pauseless FSM (no change)
not set	false	Normal FSM	Normal FSM (no change)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.28%. Comparing base (58daa99) to head (a939c13).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../helix/core/realtime/SegmentCompletionManager.java	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17852      +/-   ##
============================================
+ Coverage     63.23%   63.28%   +0.04%     
  Complexity     1466     1466              
============================================
  Files          3190     3190              
  Lines        192009   192003       -6     
  Branches      29412    29411       -1     
============================================
+ Hits         121424   121502      +78     
+ Misses        61063    60987      -76     
+ Partials       9522     9514       -8     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-11	63.22% <40.00%> (+0.01%)	⬆️
java-21	63.24% <40.00%> (+0.02%)	⬆️
temurin	63.28% <40.00%> (+0.04%)	⬆️
unittests	63.27% <40.00%> (+0.04%)	⬆️
unittests1	55.58% <ø> (+0.02%)	⬆️
unittests2	34.27% <40.00%> (+0.02%)	⬆️

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[9aman]

@noob-se7en can we update the corresponding docs, if any, an mention about the need for config for older versions.

[9aman]

I don't see any backward compatibility issues too as the existing code defaulted to using the flag to figure out the fsmClass.

[9aman]

LGTM