enhance: [2.5] Remove balance constraints between channel and segment tasks

[weiliu1031]

issue: #42176
pr: #42177

Remove the mutual exclusion constraints between channel and segment balance tasks to allow them to run concurrently.

Changes include:

• Remove permitBalanceChannel() and permitBalanceSegment() methods from RoundRobinBalancer
• Update ChannelLevelScoreBalancer, MultiTargetBalancer, RowCountBasedBalancer, and ScoreBasedBalancer to remove constraint checks
• Allow segment balance tasks to proceed even when channel balance tasks are running
• Update test cases to reflect new behavior where balance tasks no longer block each other
• Improve error handling in task executor by preferring serviceable shard leaders for segment release operations
• Add fallback logic to find latest shard leader when serviceable leader is not available

This change improves the efficiency of load balancing by removing unnecessary coordination overhead between different types of balance operations.

Signed-off-by: Wei Liu [email protected]

[mergify]

@weiliu1031

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[codecov]

Codecov Report

Attention: Patch coverage is 54.16667% with 11 lines in your changes missing coverage. Please review.

Project coverage is 80.64%. Comparing base (b7fc382) to head (14fa621).
Report is 7 commits behind head on 2.5.

Files with missing lines	Patch %	Lines
internal/querycoordv2/task/executor.go	12.50%	7 Missing ⚠️
...ernal/querycoordv2/balance/multi_target_balance.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.5   #42410      +/-   ##
==========================================
- Coverage   80.68%   80.64%   -0.04%     
==========================================
  Files        1428     1428              
  Lines      205836   205829       -7     
==========================================
- Hits       166069   165991      -78     
- Misses      33927    33999      +72     
+ Partials     5840     5839       -1     

Components	Coverage Δ
Client	79.42% <ø> (ø)
Core	70.63% <ø> (ø)
Go	82.61% <54.16%> (-0.05%)	⬇️

Files with missing lines	Coverage Δ
internal/querycoordv2/balance/balance.go	93.82% <ø> (-0.30%)	⬇️
...erycoordv2/balance/channel_level_score_balancer.go	89.77% <100.00%> (-4.02%)	⬇️
...al/querycoordv2/balance/rowcount_based_balancer.go	93.63% <100.00%> (-0.03%)	⬇️
...ernal/querycoordv2/balance/score_based_balancer.go	96.43% <100.00%> (-0.65%)	⬇️
...ernal/querycoordv2/balance/multi_target_balance.go	81.77% <0.00%> (+0.40%)	⬆️
internal/querycoordv2/task/executor.go	76.71% <12.50%> (-1.48%)	⬇️

... and 21 files with indirect coverage changes

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[xiaofan-luan]

/lgtm
/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: weiliu1031, xiaofan-luan

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