Fixes#1276
## What's happening
`oauthex` builds `defaultDiscoveryTransport` as a package-level var, so
it
runs at import time. That path calls `proxyConfigured`, which invokes
`http.DefaultTransport`'s proxy func (`http.ProxyFromEnvironment`).
net/http
reads the proxy env vars only once and caches the result, so this
import-time
call freezes the proxy config before `main()` can set `HTTPS_PROXY`.
New in v1.8.0, which is why it's a regression — just importing `.../mcp`
(which pulls in `oauthex`) triggers it, even with no use of the SDK.
## Fix
Wrap the setup in `sync.OnceValue` so it runs on first use instead of at
import. Still built once and reused.
## Testing (removed)
- Ran the issue's repro against this branch via a local `replace`:
prints
`proxy=http://127.0.0.1:9` now, `proxy=<nil>` before.
- Added a regression test in `oauthex`.
- `go test ./...` passes.
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
3b917b466c