Error when unknown API endpoint is used in testserver (#2292)

## Changes
This PR fails the acceptance test when an unknown endpoint (i.e. not
stubbed) is used. We want to ensure that all API endpoints used in an
acceptance test are stubbed and do not otherwise silently fail with a
404.

The logs on failure output include a configuration that developers can
simply copy-paste to `test.toml` to stub the missing API endpoint. It'll
look something like:
```
[[Server]]
Pattern = "<method> <path>"
Response.Body = '''
<response body here>
'''
Response.StatusCode = <response status-code here>
```


## Tests
Manually:

output.txt when an endpoint is not found: 
```
>>> [CLI] jobs create --json {"name":"abc"}
Error: No stub found for pattern: POST /api/2.1/jobs/create
```

How this renders in the test logs:
```
    --- FAIL: TestAccept/workspace/jobs/create (0.03s)
        server.go:46: 
            
            ----------------------------------------
            No stub found for pattern: POST /api/2.1/jobs/create
            
            To stub a response for this request, you can add
            the following to test.toml:
            [[Server]]
            Pattern = "POST /api/2.1/jobs/create"
            Response.Body = '''
            <response body here>
            '''
            Response.StatusCode = <response status-code here>
            ----------------------------------------
```

Manually checked that the debug mode still works.

acceptance/cmd_server_test.go

@@ -15,7 +15,10 @@ import (
 func StartCmdServer(t *testing.T) *testserver.Server {
 	server := testserver.New(t)
 
-	server.Handle("/", func(w *testserver.FakeWorkspace, r *http.Request) (any, int) {
+	// {$} is a wildcard that only matches the end of the URL. We explicitly use
+	// /{$} to disambiguate it from the generic handler for '/' which is used to
+	// identify unhandled API endpoints in the test server.
+	server.Handle("/{$}", func(w *testserver.FakeWorkspace, r *http.Request) (any, int) {
 		q := r.URL.Query()
 		args := strings.Split(q.Get("args"), " ")
 

libs/testserver/server.go

@@ -12,6 +12,7 @@ import (
 	"github.com/stretchr/testify/assert"
 
 	"github.com/databricks/cli/internal/testutil"
+	"github.com/databricks/databricks-sdk-go/apierr"
 )
 
 type Server struct {
@@ -41,13 +42,44 @@ func New(t testutil.TestingT) *Server {
 	server := httptest.NewServer(mux)
 	t.Cleanup(server.Close)
 
-	return &Server{
+	s := &Server{
 		Server:         server,
 		Mux:            mux,
 		t:              t,
 		mu:             &sync.Mutex{},
 		fakeWorkspaces: map[string]*FakeWorkspace{},
 	}
+
+	// The server resolves conflicting handlers by using the one with higher
+	// specificity. This handler is the least specific, so it will be used as a
+	// fallback when no other handlers match.
+	s.Handle("/", func(fakeWorkspace *FakeWorkspace, r *http.Request) (any, int) {
+		pattern := r.Method + " " + r.URL.Path
+
+		t.Errorf(`
+
+----------------------------------------
+No stub found for pattern: %s
+
+To stub a response for this request, you can add
+the following to test.toml:
+[[Server]]
+Pattern = %q
+Response.Body = '''
+<response body here>
+'''
+Response.StatusCode = <response status-code here>
+----------------------------------------
+
+
+`, pattern, pattern)
+
+		return apierr.APIError{
+			Message: "No stub found for pattern: " + pattern,
+		}, http.StatusNotFound
+	})
+
+	return s
 }
 
 type HandlerFunc func(fakeWorkspace *FakeWorkspace, req *http.Request) (resp any, statusCode int)