Scale the ACP subprocess waits for the slow platform too

[its-janghoon]

Third in the sequence after #47 and #49, and a different failure from either.

stdin EOF exits cleanly gives a subprocess five seconds to exit and failed at 6.5s with a bare TimeoutError — not a spawn timeout, so neither earlier fix reached it. Windows process teardown is slower and five seconds was tight enough to flake.

Every wait in the ACP test client is on a subprocess round trip, so all three bounds now go through the same scaler as the harness: the request wait, the notification wait's default, and the explicit values callers pass it.

The pattern is the finding

This is the third place a fixed millisecond figure met a slower platform — after the spawn timeout and the ready timeout. A bound written as a literal beside a subprocess is one that will eventually be too small on somebody's machine, and slowPlatform is where that correction belongs rather than in each number.

Still worth repeating from #47: these are mitigations. What actually changed diagnostically is that a subprocess timeout now reports what the child printed, where before both streams were discarded at two separate layers.

Gates

typecheck        0 errors across the monorepo
packages/redrob  3380 pass / 0 fail
acp suite        15/15