[GEN-1815] Implement closest utxo selection algorithm

[imclvr]

Title self-explanatory

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[Copilot]

Pull request overview

This PR implements a “closest-to-amount” UTXO selection algorithm in UTXOSelectionAlgorithms and adds/updates unit tests to validate both the new selection behavior and the CoinSelectionService routing flag.

Changes:

• Implemented SelectUTXOsByClosest and refactored shared argument/funding validation into a helper method.
• Added dedicated unit tests for the closest-selection algorithm, plus shared “no UTXOs” / “insufficient funds” coverage for both algorithms.
• Strengthened an existing CoinSelectionService test to ensure the “closest” path produces a non-empty, distinguishable selection.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
test/NodeGuard.Tests/Services/CoinSelectionServiceTests.cs	Tightens the closest-selection routing assertion by ensuring the expected selection is non-empty.
test/NodeGuard.Tests/Helpers/UTXOSelectionAlgorithmsTests.cs	Adds focused unit tests for the closest-selection algorithm plus shared edge-case coverage.
src/Helpers/UTXOSelectionAlgorithms.cs	Implements closest-selection and introduces shared validation logic for selection algorithms.

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[imclvr]

@daliclovr @RodriFS regarding the debate raised whether to implement this on NBXplorer, main reason I took for implementing it on NG is that, when this is called, utxos has been already selected priorly given derivation strategy that comes from the wallet.

We could add the strategy we want on there, but that's for getting all available utxos. Then, we would still be selecting by order.

We could maybe talk this offline in more detail. @RodriFS I will borrow your time 😬 .