fix: preserve caller-owned UTXO DB connection

[iamdinhthuan]

Summary

• keep UtxoDB.apply_transaction() from closing a caller-owned SQLite connection when rejecting an unauthorized mining_reward transaction
• add a regression proving callers can continue using their explicit connection after the validation failure
• preserve the existing no-extra-connection behavior for unauthorized mint attempts

Why

apply_transaction(..., conn=external_conn) accepts a caller-owned connection. The unauthorized public mining_reward guard returned False, but closed external_conn first, so callers could not inspect, roll back, or continue their transaction after a normal validation failure.

Bounty route

• [BOUNTY] Red Team UTXO Implementation — Find Bugs, Earn RTC (50-200 RTC) rustchain-bounties#2819
• Miner / payout id: iamdinhthuan

Tests

• Red before fix: python3 -m pytest node/test_utxo_db.py::TestUtxoDB::test_user_supplied_mining_reward_preserves_external_conn -q failed with sqlite3.ProgrammingError: Cannot operate on a closed database
• python3 -m pytest node/test_utxo_db.py::TestUtxoDB::test_user_supplied_mining_reward_preserves_external_conn node/test_utxo_db.py::TestUtxoDB::test_user_supplied_mining_reward_rejected_before_opening_db node/test_utxo_db.py -q -> 72 passed
• python3 -m py_compile node/utxo_db.py node/test_utxo_db.py
• git diff --check

Duplicate / safety check

• Fresh exact PR/issue searches for caller-owned connection apply_transaction mining_reward and preserve caller owned UTXO connection returned no exact public duplicate immediately before push.
• No private keys, tokens, account-private data, cookies, or credentials are included.

[github-actions]

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[iamdinhthuan]

Closing this duplicate in favor of #5572, which has the same caller-owned UTXO connection patch and is linked to bounty claim Scottcjn/rustchain-bounties#10623.

[HCIE2054]

LGTM!