[Client] Fail a request at once when its response ends unanswered

[chr-hertel]

A connection cut mid-answer left the request pending until its timeout (120s by default) - end of stream counted as done, and a truncated JSON body was only logged as "Failed to parse message".

• SSE: if the POST's stream ends, is cut or fails to read before the answer to its request arrived, the waiting call fails with a ConnectionException
• JSON: a body that fails to read or doesn't carry the answer (truncated, empty) does the same, instead of leaking the PSR stream exception or waiting

Interacts with #463: resuming via Last-Event-ID would slot in where the unanswered stream now fails - this covers what stays unresumable (no event id, JSON bodies, read errors). Status handling of #425 / #560 is untouched.

Fixes chr-hertel#65

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