fix: use max MvccTs for iterator

[PwzXxm]

issue: #37158

[mergify]

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

[codecov]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.

Project coverage is 80.85%. Comparing base (86b9c3e) to head (5ed7230).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/proxy/util.go	54.54%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #37247      +/-   ##
==========================================
- Coverage   83.15%   80.85%   -2.30%     
==========================================
  Files        1029     1319     +290     
  Lines      157251   182693   +25442     
==========================================
+ Hits       130764   147720   +16956     
- Misses      21322    29794    +8472     
- Partials     5165     5179      +14     

Components	Coverage Δ
Client	∅ <ø> (∅)
Core	66.89% <ø> (∅)
Go	83.13% <61.53%> (-0.05%)	⬇️

Files with missing lines	Coverage Δ
internal/proxy/task_query.go	80.71% <100.00%> (ø)
internal/proxy/task_search.go	75.95% <100.00%> (ø)
internal/proxy/util.go	89.77% <54.54%> (-0.26%)	⬇️

... and 311 files with indirect coverage changes

[PwzXxm]

/run-cpu-e2e

[mergify]

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

[PwzXxm]

/run-cpu-e2e

[PwzXxm]

For exact MVCC view in cluster with multiple channels, we need to pass the channel-mvccTs back to SDK. Do it in later PR.

[congqixia]

lgtm as temp solution
/lgtm

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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