Reject duplicate inputs_set UTXOs

[caydyan]

Supersedes #3386 with the same fix rebased onto the current develop branch.

prepare_inputs_set() accepted duplicate UTXOs in the caller-provided inputs_set list. The duplicated entries are returned as separate inputs, so later input selection can count the same UTXO value more than once and build a transaction with duplicate outpoints.

This PR rejects duplicate UTXOs while parsing inputs_set.

Changes:

• Tracks seen <txid>:<vout> identifiers while parsing inputs_set.
• Rejects repeated UTXOs with a clear ComposeError.
• Adds regression coverage for duplicate explicit inputs.

Tests:

• python -m pytest counterpartycore/test/units/api/composer_test.py::test_prepare_inputs_set -q
• python -m pytest counterpartycore/test/units/api/composer_test.py -q
• python -m ruff check counterpartycore/lib/api/composer.py counterpartycore/test/units/api/composer_test.py
• python -m ruff format --check counterpartycore/lib/api/composer.py counterpartycore/test/units/api/composer_test.py
• git diff --check

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