test: fix transport error flake (#26132)

`TestResolveWorkspace/TransportError` could sometimes observe an HTTP
404 from a closing test server instead of a transport error.

Make the test deterministic by injecting a failing `http.RoundTripper`,
and add `testutil.RoundTripperFunc` for reuse.

Generated by Coder Agents.

<details>
<summary>Implementation plan</summary>

# Plan: Deterministic ResolveWorkspace transport error test

## Context

`TestResolveWorkspace/TransportError` relied on closing an
`httptest.Server` before making a request. CI showed this can race with
the request path and produce an HTTP 404 instead of a transport error.
The test should inject a transport failure directly.

## Red

1. Update the transport-error case to use a custom `http.RoundTripper`
that returns an error.
2. Confirm the test fails to compile until the reusable
`testutil.RoundTripperFunc` helper exists.

## Green

1. Add `testutil.RoundTripperFunc` in `testutil/http.go`.
2. Implement `RoundTrip` so the function type satisfies
`http.RoundTripper`.
3. Add a compile-time interface assertion for the helper.
4. Update `codersdk/workspaces_test.go` to inject a client using
`testutil.RoundTripperFunc`.
5. Keep the existing assertion that transport errors do not become
`*codersdk.Error`.

## Refactor

1. Run `gofmt` on touched Go files.
2. Check import cleanup and variable names after the implementation
compiles.
3. Keep the change limited to the reusable helper and this test.

## Verification

1. `go test ./codersdk -run TestResolveWorkspace -count=100`
2. `go test ./testutil ./codersdk -run TestResolveWorkspace -count=1`
3. `git diff --check`

</details>

codersdk/workspaces_test.go

@@ -13,8 +13,10 @@ import (
 	"github.com/google/uuid"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"golang.org/x/xerrors"
 
 	"github.com/coder/coder/v2/codersdk"
+	"github.com/coder/coder/v2/testutil"
 )
 
 func TestResolveWorkspace(t *testing.T) {
@@ -271,13 +273,13 @@ func TestResolveWorkspace(t *testing.T) {
 	t.Run("TransportError", func(t *testing.T) {
 		t.Parallel()
 
-		// Close the server immediately so the transport layer fails.
-		srv := httptest.NewServer(http.NotFoundHandler())
-		srvURL, err := url.Parse(srv.URL)
+		baseURL, err := url.Parse("http://example.com")
 		require.NoError(t, err)
-		srv.Close()
-
-		client := codersdk.New(srvURL)
+		client := codersdk.New(baseURL, codersdk.WithHTTPClient(&http.Client{
+			Transport: testutil.RoundTripperFunc(func(*http.Request) (*http.Response, error) {
+				return nil, xerrors.New("transport error")
+			}),
+		}))
 
 		_, err = client.ResolveWorkspace(t.Context(), uuid.NewString())
 		require.Error(t, err)