fix: make fund locking atomic

[q36zhd46w17o]

Summary

Closes #125.

This fixes the UTXO locking race by moving the idempotency lookup, UTXO selection, pending transaction creation, and output locking into one BEGIN IMMEDIATE SQLite transaction. That keeps the critical section enforced by the wallet database instead of a process-global mutex.

It also makes lock_output() fail if the selected output is no longer UNSPENT, so a stale selection cannot silently produce a pending transaction without actually locking the output.

Bounty

This PR is submitted for the open #125 bounty: Tier S, 15,000 XTM, first merged PR wins, payment after merge. Payout details can be provided through the maintainer-approved flow if this PR is selected.

Tests

Using stable Rust 1.96.0 because current Tari dependencies require rustc >= 1.93.0:

• cargo fmt --all -- --check
• cargo test -p minotari transactions::fund_locker -- --nocapture
• cargo test -p minotari db::outputs -- --nocapture
• cargo check -p minotari
• cargo test -p minotari --lib --bins
• cargo test -p minotari --doc
• cargo clippy -p minotari --all-targets -- -D warnings

[gemini-code-assist]

Code Review

This pull request updates the fund locking mechanism to prevent race conditions and ensure idempotency under concurrent execution. Specifically, it modifies lock_output to verify that exactly one row was updated, returning an error if the output is no longer unspent. It also updates FundLocker::lock to initiate an immediate transaction earlier in the function, ensuring that idempotency checks and UTXO selection occur within the same transaction. Finally, concurrent unit tests are added to verify these behaviors. There are no review comments, so I have no feedback to provide.

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[SWvheerden]

I dont like this approach, ideally you should allow multiple lookups without locking all, and you can more grafully handleing the locking