fix: don't resend block acceptance if already sent

[brice-stacks]

The signer is currently sending an approval for every pre-commit that it receives after reaching the threshold weight. This change just makes sure it only sends its approval when it first reaches that threshold, and then ignores the rest of the pre-commits.

[brice-stacks]

It turns out this is not as simple as I hoped.

[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.63%. Comparing base (c9003aa) to head (a30265c).
⚠️ Report is 25 commits behind head on develop.

Files with missing lines	Patch %	Lines
stacks-node/src/tests/signer/v0.rs	0.00%	25 Missing ⚠️
stacks-signer/src/signerdb.rs	85.71%	2 Missing ⚠️
stacks-signer/src/v0/signer.rs	85.71%	1 Missing ⚠️

❌ Your project status has failed because the head coverage (77.63%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6750      +/-   ##
===========================================
- Coverage    77.90%   77.63%   -0.27%     
===========================================
  Files          580      580              
  Lines       361187   361131      -56     
===========================================
- Hits        281374   280371    -1003     
- Misses       79813    80760     +947     

Files with missing lines	Coverage Δ
stacks-common/src/util/macros.rs	88.54% <ø> (ø)
...tacks-node/src/tests/signer/commands/block_wait.rs	23.76% <ø> (+23.76%)	⬆️
stackslib/src/net/api/postblock_proposal.rs	67.03% <100.00%> (+0.15%)	⬆️
stacks-signer/src/v0/signer.rs	74.03% <85.71%> (-0.57%)	⬇️
stacks-signer/src/signerdb.rs	92.99% <85.71%> (+0.35%)	⬆️
stacks-node/src/tests/signer/v0.rs	16.46% <0.00%> (+1.82%)	⬆️

... and 86 files with indirect coverage changes

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