fix(ckbtc): prevent double-mint of cached UTXOs after transaction finalization

[gregorydemay]

Summary

• Prevent double-minting of UTXOs that are returned by the get_utxos cache after their transaction has been finalized and forgotten by the minter
• Filter processable UTXOs against minted_outpoints before check/mint to deduplicate already-minted outpoints
• Track deduplicated outpoints in a new deduplicated_outpoints state field with a ckbtc_minter_deduplicated_outpoint_count metric for observability
• Add a test (should_not_mint_cached_utxo) that reproduces the full scenario: mint → submit tx → cache fill → finalize tx → update_balance with stale cache

🤖 Generated with Claude Code

[THLO]

Very nice PR! Thanks!

[tmu0]

💯