Enable the signing logic to fall back to the hardware wallet if secret key is not found.

[murisi]

Describe your changes

Currently the transaction signing logic fails fall back to the hardware wallet if the secret key is not found in the wallet store. This is because an error is propagated as soon as the secret key lookup fails. This PR fixes that. Additionally, the signing code now checks that the number of public keys used in signing exceeds the given account's threshold.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes

[codecov]

Codecov Report

Attention: Patch coverage is 38.46154% with 16 lines in your changes missing coverage. Please review.

Project coverage is 72.69%. Comparing base (0d7140f) to head (24e790a).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/sdk/src/signing.rs	38.46%	16 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3730   +/-   ##
=======================================
  Coverage   72.68%   72.69%           
=======================================
  Files         338      338           
  Lines      104319   104339   +20     
=======================================
+ Hits        75826    75849   +23     
+ Misses      28493    28490    -3     

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[grarco]

LGTM, thank you!