feat: add managed_core_account_set_transaction_label FFI function

[xdustinface]

Add mutable access to FFIManagedCoreAccount via inner_mut() and expose a function to set or clear transaction labels by txid.

Also add comprehensive tests for:

• managed_core_account_get_transactions with real transaction data
• managed_core_account_free_transactions verifying no crash on free
• Label round-trip (set, read back, clear, verify cleared)
• Error cases (null account, missing txid)

Based on:

• feat: validate transaction label size #617

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[codecov]

Codecov Report

❌ Patch coverage is 95.28986% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.69%. Comparing base (734b35f) to head (4b5f8c7).
⚠️ Report is 10 commits behind head on v0.42-dev.

Files with missing lines	Patch %	Lines
key-wallet-ffi/src/managed_account.rs	94.90%	13 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           v0.42-dev     #618      +/-   ##
=============================================
+ Coverage      67.21%   67.69%   +0.47%     
=============================================
  Files            321      321              
  Lines          68249    68523     +274     
=============================================
+ Hits           45874    46384     +510     
+ Misses         22375    22139     -236     

Flag	Coverage Δ
core	75.21% <ø> (ø)
ffi	39.40% <94.90%> (+3.81%)	⬆️
rpc	19.92% <ø> (ø)
spv	83.72% <ø> (-0.04%)	⬇️
wallet	67.48% <100.00%> (+0.03%)	⬆️

Files with missing lines	Coverage Δ
...y-wallet/src/managed_account/transaction_record.rs	100.00% <100.00%> (ø)
key-wallet-ffi/src/managed_account.rs	60.40% <94.90%> (+7.26%)	⬆️

... and 18 files with indirect coverage changes

[github-actions]

This PR has merge conflicts with the base branch. Please rebase or merge the base branch into your branch to resolve them.