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>

testutil/http.go

@@ -9,6 +9,15 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// RoundTripperFunc adapts a function to an http.RoundTripper.
+type RoundTripperFunc func(*http.Request) (*http.Response, error)
+
+var _ http.RoundTripper = RoundTripperFunc(nil)
+
+func (f RoundTripperFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return f(req)
+}
+
 // RequireEventuallyResponseOK makes HTTP GET requests to the given endpoint until it returns
 // 200 OK with a valid JSON response that can be decoded into target, or until the context
 // times out. This is useful for waiting for HTTP servers to become ready during tests,