fix: Choose wrong shard leader during balance channel(#29525)

[weiliu1031]

issue: #29523
pr: #29525

readable shard leader should still be the old one during channel balance, if the new shard leader is not ready.
This PR fixed that query coord choose wrong shard leader during balance channel

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (14d8b1f) 82.14% compared to head (b0259ea) 82.13%.
Report is 17 commits behind head on 2.3.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.3   #29532      +/-   ##
==========================================
- Coverage   82.14%   82.13%   -0.02%     
==========================================
  Files         833      833              
  Lines      119556   119711     +155     
==========================================
+ Hits        98215    98321     +106     
- Misses      18124    18163      +39     
- Partials     3217     3227      +10     

Files	Coverage Δ
internal/querycoordv2/services.go	84.96% <100.00%> (+0.09%)	⬆️

... and 23 files with indirect coverage changes

[yah01]

/lgtm

[mergify]

@weiliu1031 E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[weiliu1031]

/run-cpu-e2e

[congqixia]

/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: congqixia, weiliu1031

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