Fix nil dereference in awaitResponse when the connection closes

[ABowlOfEleven]

Fixes #20.

Problem

Close() closes the response channel of every pending request. A receive on a closed channel yields nil, and awaitResponse read response.Error from it with no check. Any request that was still waiting when the connection closed (explicit Close(), a read error such as PsyNet's DuplicateLogin drop, or the pong timeout) panicked the calling goroutine.

Change

• awaitResponse checks the receive. If the channel was closed with no response, it returns ctx.Err() when the context has ended, and the new ErrConnectionClosed otherwise. The ctx.Err() branch covers the cleanup goroutine in sendRequestAsync, which closes the same channel on ctx.Done().
• ErrConnectionClosed is exported so that callers can use errors.Is to tell "reconnect and retry" apart from a PsyNet error.
• No other behavior changes. A request that gets its response, a PsyNet error, or a context timeout returns exactly what it did before.

Tests

Both new tests panic on master and pass with the fix:

• TestPsyNetRPC_CloseWithPendingRequest: waits until the request is in pendingReqs, calls rpc.Close(), expects ErrConnectionClosed.
• TestPsyNetRPC_ServerDropsConnectionWithPendingRequest: the mock server closes the connection when the request arrives, so Close() runs from readMessages. Expects ErrConnectionClosed and IsConnected() == false. This needed one small option on MockWSServer (SetCloseOnRequest).

Locally (Go 1.26.8, Windows): gofmt -l . is empty, go vet ./... is clean, the two new tests pass with -count=20, and the full suite passes with -count=3. I could not run -race on this machine (no C compiler for cgo), so that part is left to CI.

[dank]

Nice, thanks!