mcp: extend the write deadline on every write to a keep-alive stream

[jmrplens]

Fixes #1262.

http.Server.WriteTimeout is the ordinary slow-write guard, and it applies to the whole response. A subscriptions/listen stream is a response the transport means to hold open indefinitely, so the deadline passes while the stream sits idle and healthy. Nothing notices, because nothing is being written, and the first real notification after it fails and takes the stream with it.

Only the transport knows a response became a stream, so only the transport can move the deadline. As agreed in #1262, every write to a stream that runs a keep-alive (#1232) now pushes the deadline out by twice the keep-alive interval, through http.ResponseController.SetWriteDeadline:

• Extended rather than cleared, so WriteTimeout keeps meaning what it was set for, and a peer that stopped reading still fails within a bounded time.
• Twice the interval rather than once, so a keep-alive that lands a little late does not race the deadline it exists to defer.
• Scoped to streams with a keep-alive, which today means subscriptions/listen, because the interval is what the extension is measured in. A stream with no periodic write has nothing to measure it by.
• Best effort: a writer that manages its own deadlines reports ErrNotSupported, and a server with no WriteTimeout has no deadline to move, so the result is ignored.

TestStreamWriteDeadline_SurvivesServerWriteTimeout holds a listen stream open behind an httptest server whose WriteTimeout is three keep-alive intervals, for five times that timeout. Against main it fails every time: no keep-alive arrives after the 75 ms deadline, because the first write past it fails and ends the stream.