test(cli): tolerate stray requests in fake agent API (#27127)

Test_TaskSend flaked (coder/internal#1547, coder/internal#1609) when a
stray POST /chat/completions hit the fake agent API and the catch-all
handler called t.Fatalf. No code under test posts that path to the
sidebar app URL; the request most likely came from another test's
lingering client after its server's ephemeral port was reused. Fatalf
was also called off the test goroutine, which the testing package
forbids.

Unknown paths now get a 404 and a log line with request details for
attribution. Unstubbed known agentapi endpoints still fail the test, via
t.Errorf, so a coderd regression is still caught.

cli/task_test.go

@@ -3,6 +3,8 @@ package cli_test
 import (
 	"context"
 	"encoding/json"
+	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"slices"
@@ -533,6 +535,14 @@ func startFakeAgentAPI(t *testing.T, handlers map[string]http.HandlerFunc) *fake
 
 	mux := http.NewServeMux()
 
+	// requestDetail records method, path, User-Agent, and a bounded view of
+	// the request body so unexpected traffic can be attributed without
+	// unbounded logging.
+	requestDetail := func(r *http.Request) string {
+		body, _ := io.ReadAll(io.LimitReader(r.Body, 4<<10))
+		return fmt.Sprintf("method=%s path=%s user-agent=%q body=%q", r.Method, r.URL.Path, r.UserAgent(), body)
+	}
+
 	// Register all provided handlers with call tracking
 	for path, handler := range handlers {
 		mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
@@ -543,18 +553,27 @@ func startFakeAgentAPI(t *testing.T, handlers map[string]http.HandlerFunc) *fake
 		})
 	}
 
+	// Known agentapi endpoints without a handler fail the test: a coderd
+	// regression that calls an endpoint the test did not stub must be
+	// caught. The 404 also gives the client a well-formed response instead
+	// of leaving it hanging.
 	knownEndpoints := []string{"/status", "/messages", "/message"}
 	for _, endpoint := range knownEndpoints {
 		if handlers[endpoint] == nil {
 			endpoint := endpoint // capture loop variable
 			mux.HandleFunc(endpoint, func(w http.ResponseWriter, r *http.Request) {
-				t.Fatalf("unexpected call to %s %s - no handler defined", r.Method, endpoint)
+				t.Errorf("unexpected call to agentapi endpoint %s with no handler defined: %s", endpoint, requestDetail(r))
+				w.WriteHeader(http.StatusNotFound)
 			})
 		}
 	}
-	// Default handler for unknown endpoints should cause the test to fail.
+	// Unknown paths get a 404 and a log line, but do not fail the test.
+	// Stray traffic can arrive here, most likely from another test's
+	// lingering client whose closed server's ephemeral port was reused by
+	// this one, so failing on unknown paths would create false flakes.
 	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
-		t.Fatalf("unexpected call to %s %s - no handler defined", r.Method, r.URL.Path)
+		t.Logf("unexpected request to unknown path, likely cross-test chatter from a reused ephemeral port: %s", requestDetail(r))
+		w.WriteHeader(http.StatusNotFound)
 	})
 
 	fake.server = httptest.NewServer(mux)