[GEN-1815] Create FF for coin selection closest strategy

[imclvr]

FF for enabling new utxo selection from NG.

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[Copilot]

Pull request overview

Adds an environment-driven feature flag to enable/disable a new “closest amount” UTXO coin selection strategy from NodeGuard configuration, with local dev defaults set to disabled.

Changes:

• Introduces ENABLE_COIN_SELECTION_BY_CLOSEST feature flag in Constants and wires it to ENABLE_COIN_SELECTION_BY_CLOSEST environment variable parsing.
• Adds the new environment variable (default "false") to local launch configurations for both dotnet and VS Code.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/Properties/launchSettings.json	Adds ENABLE_COIN_SELECTION_BY_CLOSEST=false to local dev environment variables.
src/Helpers/Constants.cs	Defines and initializes the new feature flag from environment variables.
.vscode/launch.json	Adds ENABLE_COIN_SELECTION_BY_CLOSEST=false to VS Code debug environment variables.

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