fix: apply MTU to multi-path routes in routetable

[Copilot]

bestTarget.MTU was silently dropped for multi-path routes because kernRoute.MTU was only assigned inside the single-path else branch.

Change

Move MTU into the base kernelRoute struct initialisation so it is set regardless of whether the route is single- or multi-path:

// Before
kernRoute := kernelRoute{
    ...
    Protocol: proto,
    // MTU absent here
}
if len(bestTarget.MultiPath) > 0 {
    // MTU never reaches the kernel for multi-path routes
} else {
    kernRoute.MTU = bestTarget.MTU  // only single-path got MTU
}

// After
kernRoute := kernelRoute{
    ...
    Protocol: proto,
    MTU:      bestTarget.MTU,  // applies to both single- and multi-path
}

Test

Added "Should program MTU on a multi-path route and correct an incorrect MTU on resync" to the existing multi-path route test suite. The test:

1. Programs a multi-path route with MTU=1400 and verifies it is set on the kernel route.
2. Corrupts the MTU to 9000 in the mock dataplane behind the RouteTable's back.
3. Triggers a full resync and verifies the correct MTU is restored.