fix: improve performance of ApplyPowerTableDiffs in ImportSnapshotToDatastore

[hanabi1224]

The sort operation in ApplyPowerTableDiffs turns out to be expensive.
This PR introduces another func ApplyPowerTableDiffsToMap(powerTableMap map[gpbft.ActorID]gpbft.PowerEntry, diffs ...PowerTableDiff) (map[gpbft.ActorID]gpbft.PowerEntry, error) to avoid sorting.

It also adds more validations against first instance, latest instance and powertable cid.

Perf stats of importing a mainnet F3 snapshot on my laptop show ~99% perf gain

# PR
0.884s
0.773s
0.864s

# main
86.547s
88.422s
86.998s

[codecov]

Codecov Report

❌ Patch coverage is 53.96825% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.46%. Comparing base (9288fba) to head (c04cb35).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
certstore/snapshot.go	30.95%	18 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1047      +/-   ##
==========================================
- Coverage   64.77%   64.46%   -0.31%     
==========================================
  Files          81       81              
  Lines        9896     9942      +46     
==========================================
- Hits         6410     6409       -1     
- Misses       2961     2999      +38     
- Partials      525      534       +9     

Files with missing lines	Coverage Δ
certs/certs.go	94.28% <100.00%> (+0.34%)	⬆️
certstore/snapshot.go	41.61% <30.95%> (-6.22%)	⬇️

... and 8 files with indirect coverage changes

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[Copilot]

Pull Request Overview

This PR optimizes the performance of applying power table diffs during F3 snapshot import by introducing a new map-based approach that avoids expensive sorting operations on each iteration. The optimization shows a ~99% performance improvement when importing mainnet F3 snapshots.

• Introduced ApplyPowerTableDiffsToMap function to work with power table maps instead of sorted arrays
• Added comprehensive validation for instance numbers, latest instance bounds, and power table CID verification
• Refactored importSnapshotToDatastoreWithTestingPowerTableFrequency to use the new map-based approach and include additional safety checks

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
certstore/snapshot.go	Refactored snapshot import logic to use map-based power table operations and added validation checks
certs/certs.go	Added new map-based power table diff functions and helper functions for array-map conversion

[Kubuxu]

SGTM apart from the two issues highlighted by Copilot