chore(nat): Improve AllocatorError variants and messages

[qmonnet]

AllocatorError::NoFreePort is not about no port block being available, but not ports being available in a block. The message is confusing: let's fix it. It's been so confusing that we used the error at a location where the error is not related (passing it an incorrect argument): let's add a new variant for that location, to make it easier to troubleshoot errors.

[Copilot]

Pull request overview

This PR aims to reduce confusion when diagnosing stateful NAT allocator failures by refining AllocatorError variants and improving associated error messages.

Changes:

• Clarifies the AllocatorError::NoFreePort error message to reflect “no free port in a block” (not “no free block”).
• Introduces a new AllocatorError::PortReservationFailed variant for port reservation failures.
• Updates stateful NAT error-to-DoneReason translation to account for the new variant.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
nat/src/stateful/allocation.rs	Updates allocator error messages and adds PortReservationFailed.
nat/src/stateful/mod.rs	Extends translate_error to handle the new allocator error variant.

Comments suppressed due to low confidence (1)

nat/src/stateful/mod.rs:517

confidence: 7
tags: [logic]

`AllocatorError::PortReservationFailed(_)` is currently translated to `DoneReason::NatFailure`, but reservation failures are typically a resource/collision condition similar to `NoFreePort(_)` (which maps to `NatOutOfResources`). If `PortReservationFailed` is meant to represent “requested port already in use / not available”, consider translating it to `NatOutOfResources` for consistent drop reason semantics (or add a brief rationale for why this should be `NatFailure`).

    | StatefulNatError::AllocationFailure(
        AllocatorError::PortAllocationFailed(_)
        | AllocatorError::PortReservationFailed(_)
        | AllocatorError::UnsupportedIcmpCategory
        | AllocatorError::MissingDiscriminant
        | AllocatorError::UnsupportedDiscriminant,
    ) => DoneReason::NatFailure,

</details>