F# codegen: AsyncMode-aware abort + terminal IResult endpoint (GH-2969)

[jeremydmiller]

A route-bound endpoint whose handler returns IResult compiles to AsyncMode.ReturnFromLastNode — the IResult's ExecuteAsync is the method's returned Task, with no task { } block. In that mode every branch of the body must itself be a Task, so the route-guard's abort can't be () — it must be Task.CompletedTask. This PR makes the abort expression AsyncMode-aware and adds the endpoint that exercises it.

Changes

• FSharpEmitHelpers.AbortExpression(method) — () inside a task { } body or a synchronous-Task method (None, where the machinery appends a trailing CompletedTask), but Task.CompletedTask for ReturnFromLastNode. WriteAbortGuard and ReadHttpFrame's route-value 404 guards now use it instead of a bare ().
• Fixture: GET /fsharp/result/{id} returns IResult with a route value, exercising the ReturnFromLastNode abort (a missing id yields Task.CompletedTask). The terminal IResult is executed directly — matching the C# path (no MaybeEnd guard for a terminal result).

Generated F#

override this.Handle(httpContext: HttpContext) : Task =
    let id = (httpContext.GetRouteValue("id") :?> string)
    if isNull id then
        httpContext.Response.StatusCode <- 404
        System.Threading.Tasks.Task.CompletedTask   // <- was () before; ReturnFromLastNode needs Task
    else
        let result = thingEndpoints.GetResult(id)
        result.ExecuteAsync(httpContext)

Deferred

MaybeEndWithResultFrame F# emit (the WolverineContinue short-circuit for intermediate IResults) is intentionally not done here: that frame is added by ResultContinuationPolicy during a full MapWolverineEndpoints compile, which the no-host ChainFor fixture harness doesn't run — so it can't be exercised until the behavioural (host-based) harness lands. Documented inline.

Verification

• Both F# gates pass; GET /fsharp/result/{id} renders + compiles.
• Full wolverine.slnx Release regression — 0 warnings, 0 errors.

Part of #2969.

🤖 Generated with Claude Code