[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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