feat(v3): auto-fill channel parameters from message location (#139)

[lerenn]

Issue

Fixes #139 — a channel parameter's location runtime expression was parsed but never used; the parameter always had to be passed manually when sending.

Change

When a channel parameter declares a location ($message.payload#/... or $message.header#/...), the generated Send/Request functions now derive it from the outgoing message when the caller leaves it empty, while keeping it overridable:

// Fill UserId from the message ($message.payload#/userId) if not set
if params.UserId == "" {
    if v := msg.Payload.UserId; v != nil {
        params.UserId = *v
    }
}
addr := fmt.Sprintf("v3.issue139.user.%s", params.UserId)

This reuses the existing location-tree machinery (createTreeUntilLocation, referenceToStructAttributePath) already used for correlation IDs and reply addresses. Operation.setParametersLocationDependencies resolves the targeted field on the operation message and records whether it is required (pointer vs. value). A Follow() method was added to Parameter. The feature is send-side only (on receive the message does not exist yet).

Because no existing spec used parameter location, go generate ./... produces no golden-file diff outside the new test.

Test

test/v3/issues/139 sends through a capturing mock broker (no real broker needed) and asserts the published address is v3.issue139.user.abc when userId is left empty (filled from the payload), and ...override when set explicitly. The first assertion fails before the change (empty parameter ⇒ v3.issue139.user.).

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