fix: avoid livelock after mining delay

[brice-stacks]

If, somehow, we arrive in a scenario where the miner's proposals are not reaching signers (or at least not 70% of them) for block_proposal_max_age_secs, then finally, the proposals arrive at the signers, they would previously silently drop this proposal, neither approving or rejecting it. The miner, continuing to wait for approval or rejection would be permanently stuck in the propose_block loop, only ever reproposing the same block. This commit changes the signer behavior so that instead of silently ignoring the block, the reject it with a new reason, ProposalTooOld, which, when it receives >= 30% of these rejections, will trigger the miner to exit that loop and mine a new block.

Checklist

• Test coverage for new or modified code paths
• For new Clarity features or consensus changes, add property tests (see docs/property-testing.md)
• Changelog fragment(s) or "no changelog" label added (see changelog.d/README.md)
• Required documentation changes (e.g., rpc/openapi.yaml for RPC endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo

[coveralls]

Coverage Report for CI Build 31036439263

Coverage increased (+0.06%) to 86.642%

Details

• Coverage increased (+0.06%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (9 of 10 lines covered, 90.0%).
• 387 coverage regressions across 36 files.

Uncovered Changes

File	Changed	Covered	%
libsigner/src/v0/messages.rs	6	5	83.33%
Total (2 files)	10	9	90.0%

Coverage Regressions

387 previously-covered lines in 36 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
stackslib/src/chainstate/burn/db/sortdb.rs	229	90.46%
stackslib/src/burnchains/db.rs	33	78.24%
stacks-signer/src/client/stacks_client.rs	27	86.76%
stackslib/src/chainstate/stacks/db/snapshot/blocks.rs	9	96.04%
stacks-signer/src/monitoring/mod.rs	9	77.08%
stackslib/src/chainstate/stacks/db/snapshot/burnchain.rs	7	94.77%
stackslib/src/burnchains/bitcoin/network.rs	5	79.8%
stackslib/src/chainstate/nakamoto/mod.rs	5	84.77%
stackslib/src/chainstate/stacks/db/snapshot/common.rs	5	96.75%
stackslib/src/chainstate/stacks/db/snapshot/index.rs	5	97.62%

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Coverage Stats

Relevant Lines:	232774
Covered Lines:	201681
Line Coverage:	86.64%
Coverage Strength:	19357119.01 hits per line

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💛 - Coveralls