fix(key-wallet): return the fee the tx actually pays from build/build_signed

[QuantumExplorer]

Fixes #871

Problem

TransactionBuilder::build_unsigned() / build_signed() returned a fee computed from the encoded size of the built transaction (fee_rate.calculate_fee(encoded_size(tx))). When the builder drops a dust change remainder (0 < change ≤ 546 duffs), those duffs go to miners — the transaction's real fee is the size fee plus the dust, so the returned value under-reported by up to 546 duffs exactly in the case where the fee got larger. Callers surfacing this value as "the network fee" (e.g. send_payment via FFI/Swift, see dashpay/platform#4049) showed a wrong number.

Fix

Return Σ(selected input values) − Σ(output values) — the fee the transaction actually pays — from both build_unsigned() and build_signed(). Both totals are already in scope, so this removes code rather than adding lookups:

• build_unsigned(): fee computed from the assembled tx's inputs/outputs.
• build_signed(): input total captured before signing consumes the UTXO list; output total read from the signed tx.
• The now-unused encoded_size helper, its error paths (including the reservation-release-on-encode-failure branches), and the Encodable import are removed.
• Doc comments updated to state the new semantics.

The semantics are unchanged in the normal cases: with emitted change or a drain (SelectionStrategy::All), inputs − outputs equals the size-based estimate. Only the dust-drop case changes, where the new value is the correct one.

Consumer audit

Internal consumers pass the fee through unchanged: key-wallet-ffi (transaction.rs fee_out), AssetLockResult::fee, and transaction_building.rs wrappers. The dash-spv integration tests assert fees via balance deltas, which already match the corrected semantics.

Testing

• New regression test test_dropped_dust_change_counts_toward_returned_fee: 150 526-duff input, 150 000-duff output → 300-duff dust dropped, returned fee is 526 (not the 226 size fee).
• cargo test -p key-wallet — 526 passed
• cargo test -p key-wallet-ffi — all passed
• cargo test -p dash-spv --test dashd_sync transaction against a real dashd regtest node — 9 passed
• cargo clippy -p key-wallet --all-features --all-targets and cargo fmt --check — clean

Follow-up

dashpay/platform can now delete the send_payment caller-side recomputation block and take build_signed's fee directly (pinned by the regression tests added in dashpay/platform#4049).

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved transaction fee reporting to reflect the actual fee deducted from selected inputs and outputs.
  • Included dropped dust change amounts in the reported transaction fee.
• Reliability
  • Added a limit of 500 inputs during transaction assembly to support safer transaction building.

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Review details

⚙️ Run configuration

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Review profile: CHILL

Plan: Pro

Run ID: 9e7d1a2d-e9e2-4012-b7f3-62ceb271f854

📥 Commits

Reviewing files that changed from the base of the PR and between bfb02ad and 2e4c1c4.

📒 Files selected for processing (1)

• key-wallet/src/wallet/managed_wallet_info/transaction_builder.rs

📝 Walkthrough

Walkthrough

TransactionBuilder now reports actual transaction fees as total selected inputs minus final outputs. Both unsigned and signed builds use this calculation, and a regression test verifies that dropped dust change is included in the returned fee.

Changes

Actual transaction fee reporting

Layer / File(s)	Summary
Input-output fee calculation key-wallet/src/wallet/managed_wallet_info/transaction_builder.rs	Unsigned and signed transaction builds return total inputs minus final outputs instead of size-based fee estimates; encoding errors are mapped to BuilderError::InvalidData.
Dropped dust fee regression key-wallet/src/wallet/managed_wallet_info/transaction_builder.rs	A unit test verifies that omitted dust change contributes to the returned fee.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly states the main change: returning the actual fee from build/build_signed.
Linked Issues check	✅ Passed	The fee calculation now matches the issue by using selected inputs minus final outputs, with a regression test for dust-drop behavior.
Out of Scope Changes check	✅ Passed	The changes stay focused on fee semantics, documentation, and a regression test, with only minor supporting cleanup.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/rust-dashcore-871-e5e978

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.74%. Comparing base (d4692bb) to head (2e4c1c4).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #872      +/-   ##
==========================================
+ Coverage   73.67%   73.74%   +0.07%     
==========================================
  Files         325      325              
  Lines       73775    73799      +24     
==========================================
+ Hits        54353    54424      +71     
+ Misses      19422    19375      -47     

Flag	Coverage Δ
core	77.08% <ø> (ø)
ffi	47.36% <ø> (ø)
rpc	20.00% <ø> (ø)
spv	91.05% <ø> (+0.21%)	⬆️
wallet	73.33% <100.00%> (+0.08%)	⬆️

Files with missing lines	Coverage Δ
.../wallet/managed_wallet_info/transaction_builder.rs	87.59% <100.00%> (+1.93%)	⬆️

... and 4 files with indirect coverage changes

[QuantumExplorer]

Reviewed