fix: [2.5] Fix standby mixcoord start failed

[bigsheeper]

When standby transitions to active, the component state changes to Initialize. If the initialization takes too long (exceeding the liveness probe's maximum retries), the standby pod is stopped and fails to start.
This PR removes the Initialize state during standby transitions in rolling upgrades. The state now switches directly from standby to healthy, preventing health check failures.

issue: #37630

pr: #38308

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[mergify]

@bigsheeper go-sdk check failed, comment rerun go-sdk can trigger the job again.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.10%. Comparing base (accfacf) to head (4c1004d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #38914      +/-   ##
==========================================
- Coverage   81.13%   81.10%   -0.03%     
==========================================
  Files        1385     1385              
  Lines      196070   196074       +4     
==========================================
- Hits       159083   159035      -48     
- Misses      31397    31436      +39     
- Partials     5590     5603      +13     

Components	Coverage Δ
Client	79.12% <ø> (ø)
Core	69.45% <ø> (-0.02%)	⬇️
Go	83.07% <100.00%> (-0.03%)	⬇️

Files with missing lines	Coverage Δ
internal/datacoord/server.go	73.31% <100.00%> (+0.09%)	⬆️
internal/querycoordv2/server.go	79.19% <100.00%> (+0.02%)	⬆️
internal/rootcoord/root_coord.go	88.10% <100.00%> (ø)

... and 27 files with indirect coverage changes