Fix: Ensure toXOnly always returns Uint8Array

[Dairus01]

Fix: Ensure toXOnly Always Returns Uint8Array (Fixes #2249)

This PR resolves issue #2249 by updating the toXOnly function so it consistently returns a Uint8Array, regardless of whether the input is a Buffer or a Uint8Array.

Previously, when a Buffer was passed into toXOnly, the function returned a Buffer. This created inconsistencies, especially when users relied on behavior like calling .equals() on the returned value, assuming it would always be a Buffer. In some environments the function could instead return a sliced Uint8Array, causing unexpected failures. Code relying on strict Uint8Array behavior also ran into conflicts when a Buffer was preserved.

By enforcing return new Uint8Array(...) for all cases, we guarantee:

• Consistency: The output type is always Uint8Array.
• Safety: No accidental inheritance of Buffer specific methods like .equals or .toString, which may not exist for Uint8Array or in certain runtimes.

Changes

• Updated ts_src/psbt/bip371.ts so that toXOnly always returns a new Uint8Array, both for 32 byte inputs and for slices.
• Updated test/bip371.spec.ts:
  • Replaced assert.strictEqual with assert.deepStrictEqual to check content equality rather than reference equality.
  • Added assert.notStrictEqual to ensure a fresh instance is created and input Buffer identity is never preserved.

Related Issue

Fixes #2249

Verification

• Executed the reproduce_issue.ts script created for this bug and confirmed that xOnly.equals is no longer defined, eliminating the type confusion described in the issue report.
• Ran npm run nobuild:unit and verified that the full test suite passes with no regressions.