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

This pull-request has been approved by: bigsheeper
To complete the pull request process, please assign wxyucs after the PR has been reviewed.
You can assign the PR to them by writing /assign @wxyucs in a comment when ready.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• internal/datacoord/OWNERS
• internal/querycoordv2/OWNERS
• internal/rootcoord/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.07%. Comparing base (9a63383) to head (253b938).
Report is 8 commits behind head on 2.5.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.5   #39240   +/-   ##
=======================================
  Coverage   81.07%   81.07%           
=======================================
  Files        1394     1394           
  Lines      196509   196576   +67     
=======================================
+ Hits       159313   159381   +68     
+ Misses      31595    31586    -9     
- Partials     5601     5609    +8     

Components	Coverage Δ
Client	78.26% <ø> (ø)
Core	69.53% <ø> (ø)
Go	83.04% <91.13%> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
internal/datacoord/server.go	73.58% <100.00%> (+0.12%)	⬆️
internal/querycoordv2/server.go	79.00% <100.00%> (+0.05%)	⬆️
internal/rootcoord/root_coord.go	90.08% <100.00%> (-0.12%)	⬇️

... and 34 files with indirect coverage changes

[bigsheeper]

/hold