[fix][misc] Fix Backoff mandatory-stop timer tracking

[Denovo1998]

Fixes: https://github.com/apache/pulsar/pull/25278/changes#r2878215799

Main Issue: #xyz

PIP: #xyz

Motivation

Backoff's mandatory-stop timer can be unintentionally reset when current == initial, which happens with constant backoff (initial == max) and after reduceToHalf() brings next back to initial. This prevents mandatoryStopMade from ever being reached and can cause retries to run longer than the configured mandatory stop.

Modifications

• Track the mandatory-stop start time with an explicit flag so it is set only once per backoff cycle.
• Reset the mandatory-stop timer state in reset().

Verifying this change

• Make sure that the change passes the CI checks.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: apache#25282

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.68%. Comparing base (9937d22) to head (96d2898).
⚠️ Report is 118 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master      #23       +/-   ##
=============================================
+ Coverage     30.86%   72.68%   +41.81%     
- Complexity       51    34073    +34022     
=============================================
  Files          1862     1959       +97     
  Lines        146347   155552     +9205     
  Branches      17011    17741      +730     
=============================================
+ Hits          45171   113056    +67885     
+ Misses        94161    33483    -60678     
- Partials       7015     9013     +1998     

Flag	Coverage Δ
inttests	25.75% <100.00%> (-0.83%)	⬇️
systests	22.37% <36.36%> (-0.51%)	⬇️
unittests	73.64% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...in/java/org/apache/pulsar/common/util/Backoff.java	98.11% <100.00%> (+28.72%)	⬆️
...ain/java/org/apache/pulsar/io/kinesis/Backoff.java	87.17% <100.00%> (+87.17%)	⬆️

... and 1545 files with indirect coverage changes

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