An ACP client talks to a CLI over subprocess stdio: it returns a plain
completion object rather than an iterable stream, and it does not implement
the Responses API surface. Both exclusions spelled out `acp://copilot`, so the
next ACP client silently inherited the wrong defaults — a Responses upgrade
its shim cannot serve, and a streaming call that tries to iterate a
`SimpleNamespace`.
Match on the `acp://` scheme instead. `acp+tcp://` was already handled this
way; copilot-acp's behaviour is unchanged, and the new tests pin that a
non-ACP URL still upgrades, so this is not a blanket opt-out.
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