fix: 16750 Fixed serialization for ISSTestingToolState

[imalygin]

This PR fixes the following tests:

ISS-catastrophic-1k-5
ISS-recoverable-1k-5m

There were two causes of the failure:

1. Absent state serialization
2. NPE in ISSTestingToolState#findLargestPartition.

Both issues are resolved.

Fixes #16750

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.02% (target: -1.00%)	✅ 0.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (32089ae)	97522	63704	65.32%
Head commit (807f4e6)	97551 (+29)	63704 (+0)	65.30% (-0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#16826)	30	0	0.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.

Project coverage is 63.60%. Comparing base (32089ae) to head (807f4e6).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
...java/com/swirlds/demo/iss/ISSTestingToolState.java	0.00%	30 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop   #16826      +/-   ##
=============================================
- Coverage      63.62%   63.60%   -0.02%     
  Complexity     20401    20401              
=============================================
  Files           2537     2537              
  Lines          94757    94786      +29     
  Branches        9904     9907       +3     
=============================================
  Hits           60288    60288              
- Misses         30863    30892      +29     
  Partials        3606     3606              

Files with missing lines	Coverage Δ
...java/com/swirlds/demo/iss/ISSTestingToolState.java	0.00% <0.00%> (ø)