conformance: grpc.DefaultClient is not safe for concurrent SendRPC and is not reusable after Close()

[lexfrei]

What happened:

conformance/utils/grpc.DefaultClient (the default Options.GRPCClient) has two robustness bugs, both reproduced locally:

1. Not concurrency-safe. ensureConnection/resetConnection read and write c.Conn with no lock, and SendRPC reads it (pb.NewGrpcEchoClient(c.Conn)). go test -race on N concurrent SendRPC calls against one shared client reports a data race:

WARNING: DATA RACE
Write at ... ensureConnection() grpc.go:164   (c.Conn, err = grpc.NewClient(...))
Previous read at ... grpc.go:155              (if c.Conn != nil)
Previous read at ... grpc.go:201              (pb.NewGrpcEchoClient(c.Conn))

2. Not reusable after Close(). Close() shuts the *grpc.ClientConn but does not nil c.Conn, so a later SendRPC short-circuits in ensureConnection and dials the closed connection. Reuse-after-Close() returns codes.Canceled; a fresh client returns OK against the same server.

What you expected to happen:

The documented default client should be safe to use concurrently, and either reusable after Close() or clearly single-use.

How to reproduce it (as minimally and precisely as possible):

(1) go test -race with N goroutines calling SendRPC on one DefaultClient. (2) SendRPC → Close() → SendRPC against an in-process echo server: the second call returns Canceled while a fresh client returns OK. I have both as Go tests, ready to include.

Anything else we need to know?:

Both are latent today (the suite constructs a fresh DefaultClient per request and rarely reuses one after Close()), but:

• the reuse-after-Close() bug is the root of the "helper closes a caller-supplied client it then reuses" class handled at the call site in Route the GRPCRouteWeight sampler through the injectable GRPCClient #4937 / conformance: MakeRequestAndExpectEventuallyConsistentResponse closes a caller-supplied gRPC client it does not own #4938 — nil-ing c.Conn in Close() removes it at the source;
• the concurrency bug is why a shared DefaultClient cannot currently back the Options.GRPCClient injection point the way RoundTripper is defaulted in the suite constructor.

Suggested fix: guard c.Conn with a sync.Mutex (concurrency); set c.Conn = nil in Close() (reuse). I have a fix plus the two regression tests ready — would you prefer them as one PR or two separate PRs (one per bug)? Happy either way.

/kind bug
/area conformance-machinery