fix: retry on ParentNotFound when mock-mining

[brice-stacks]

Description

When mock-mining, this error is okay and should resolve on retry once the parent block is available.

I'm not really sure how to test this scenario, so there is no test. I know this problem exists in a running mock miner and I will keep an eye on the problem once this fix is released.

Applicable issues

• fixes Mock miner not mining sometimes #6172

Checklist

• Test coverage for new or modified code paths
• Changelog is updated
• Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.33%. Comparing base (6116402) to head (ee1c3b1).

Files with missing lines	Patch %	Lines
stacks-node/src/nakamoto_node/miner.rs	0.00%	5 Missing ⚠️

❌ Your project check has failed because the head coverage (69.33%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (6116402) and HEAD (ee1c3b1). Click for more details.

HEAD has 18 uploads less than BASE

Flag	BASE (6116402)	HEAD (ee1c3b1)
	69	51

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6780      +/-   ##
===========================================
- Coverage    77.52%   69.33%   -8.19%     
===========================================
  Files          582      582              
  Lines       361565   361570       +5     
===========================================
- Hits        280294   250704   -29590     
- Misses       81271   110866   +29595     

Files with missing lines	Coverage Δ
stacks-node/src/nakamoto_node/miner.rs	0.00% <0.00%> (-76.84%)	⬇️

... and 298 files with indirect coverage changes

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