Make WeightBounds return Result<Weight, XcmError> to surface XCM weight failures

[raymondkfcheung]

This PR continues the work in #8535 and partially addresses #8419 by updating the return types in WeightBounds (specifically weight and instr_weight) to return Result<Weight, XcmError>. This allows downstream callers to access detailed error context for XCM weight calculation failures.

Why change

Previously, XCM weight calculation errors were surfaced only as Result<Weight, ()>, losing valuable error information and forcing all upstream consumers to treat them as opaque failures. This blocked structured error handling and obscured debugging, especially in message preparation and error handler setup.

How change

• Changes WeightBounds::weight and instr_weight to return Result<Weight, XcmError> instead of Result<Weight, ()>.
• Ensures consumers (e.g., XcmExecutor, ExecuteXcm) propagate or handle these structured errors correctly.
• Builds on Make WeightBounds return XcmError to surface failures #8535, which first converted internal logic to return XcmError and added tracing logs.

[raymondkfcheung]

/cmd prdoc --audience runtime_dev --bump patch

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/15066385536
Failed job name: test-linux-stable-no-try-runtime

[raymondkfcheung]

Merged into #8535 in favour of consolidating the changes there.