fix(collator): fix validation for already skipped blocks

[Rexagon]

There was a possibility that an already skipped block could be later used to start a validation task. It causes some races in tests and might as well waste resources on lagging nodes.

[github-actions]

🧪 Network Tests

To run network tests for this PR, use:

gh workflow run network-tests.yml -f pr_number=1112

Available test options:

• Run all tests: gh workflow run network-tests.yml -f pr_number=1112
• Run specific test: gh workflow run network-tests.yml -f pr_number=1112 -f test_selection=ping-pong

Test types: destroyable, ping-pong, one-to-many-internal-messages, fq-deploy, nft-index, persistent-sync

Results will be posted as workflow runs in the Actions tab.

[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.89%. Comparing base (5e6b378) to head (d0c1901).

Files with missing lines	Patch %	Lines
collator/src/validator/impls/std_impl/session.rs	72.22%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1112      +/-   ##
==========================================
- Coverage   59.89%   59.89%   -0.01%     
==========================================
  Files         484      484              
  Lines       82715    82726      +11     
  Branches    82715    82726      +11     
==========================================
+ Hits        49541    49545       +4     
- Misses      30826    30832       +6     
- Partials     2348     2349       +1     

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[SmaGMan]

lgtm