`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>
This PR adds a readiness wait to OAuth2 metadata endpoint tests to avoid rare races with server startup. Instead of immediately making HTTP requests, the tests now use `testutil.Eventually` to retry the requests until they succeed, with a short interval between attempts. This helps prevent flaky tests that might fail due to timing issues during server initialization.
Fixes: https://github.com/coder/internal/issues/996
Cian Johnston