refactor: clean op signer

[fdefelici]

Description

BurnchainOpSigner is consistently configured to be disposed of explicitly via the BurnchainOpSigner::dispose() method.
Therefore, the logic for automatically disposing of it during signing operations can be safely removed.

Also add test coverage for BurnchainOpSigner

Applicable issues

• fixes refactor: BurnchainOpSigner cleanup #6248

Additional info (benefits, drawbacks, caveats)

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
• New integration test(s) added to bitcoin-tests.yml

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 82.04%. Comparing base (381ee9b) to head (ea2c239).
Report is 16 commits behind head on develop.

Files with missing lines	Patch %	Lines
stacks-node/src/tests/epoch_21.rs	0.00%	4 Missing ⚠️
stacks-node/src/main.rs	0.00%	2 Missing ⚠️
stacks-node/src/keychain.rs	50.00%	1 Missing ⚠️
stacks-node/src/operations.rs	98.07%	1 Missing ⚠️
stacks-node/src/tests/signer/v0.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6249      +/-   ##
===========================================
- Coverage    82.10%   82.04%   -0.06%     
===========================================
  Files          546      546              
  Lines       347235   347268      +33     
  Branches       323      323              
===========================================
- Hits        285088   284933     -155     
- Misses       62139    62327     +188     
  Partials         8        8              

Files with missing lines	Coverage Δ
...-node/src/burnchains/bitcoin_regtest_controller.rs	76.88% <ø> (-0.16%)	⬇️
stacks-node/src/tests/nakamoto_integrations.rs	86.73% <100.00%> (-0.25%)	⬇️
stacks-node/src/tests/neon_integrations.rs	69.02% <100.00%> (+3.44%)	⬆️
stacks-node/src/keychain.rs	81.23% <50.00%> (ø)
stacks-node/src/operations.rs	93.18% <98.07%> (+20.45%)	⬆️
stacks-node/src/tests/signer/v0.rs	38.04% <0.00%> (-1.69%)	⬇️
stacks-node/src/main.rs	0.00% <0.00%> (ø)
stacks-node/src/tests/epoch_21.rs	0.00% <0.00%> (ø)

... and 40 files with indirect coverage changes

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[Jiloc]

lgtm!

[fdefelici]

I had to manage a merge conflict, so re-approval is needed @rdeioris @Jiloc

[Jiloc]

lgtm!