fix(ed25519): Add a check for NativeAmount to be non-negative

[sergei-boiko-trustwallet]

This pull request improves validation for XRP amounts in the NativeAmount type by adding a check to prevent negative values, and updates the associated tests to cover this new validation.

Validation improvements:

• Added a check in the NativeAmount::new constructor to return an error if a negative amount is provided, ensuring that XRP amounts cannot be negative.

Testing:

• Updated the test suite to include a case that verifies an error is returned when attempting to parse a negative amount.

[Copilot]

Pull request overview

This pull request adds validation to prevent negative XRP amounts in the NativeAmount type. The validation ensures that XRP native amounts conform to the XRP ledger specification, which requires amounts to be non-negative. This is consistent with the encoding logic that sets a positive sign bit when serializing amounts.

Changes:

• Added a negative value check in NativeAmount::new to reject amounts less than zero
• Added a test case to verify negative amounts are properly rejected

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Binary size comparison

➡️ aarch64-apple-ios: 14.34 MB

➡️ aarch64-apple-ios-sim: 14.34 MB

➡️ aarch64-linux-android:

- 18.77 MB
+ 18.77 MB 	 +1 KB

➡️ armv7-linux-androideabi:

- 16.20 MB
+ 16.20 MB 	 +1 KB

➡️ wasm32-unknown-emscripten:

- 13.68 MB
+ 13.68 MB 	 +1 KB