fix(acp-server): surface provider errors on session/prompt

[jangjoe]

Resolve #3107

session/prompt used to silently resolve with {stopReason: 'end_turn'}
when the model provider returned a transport / status failure
(rate limit, overload, 5xx, connection loss, etc.). The engine already
classifies these as born-coded Error2 instances with codes like
provider.api_error / provider.overloaded /
provider.connection_error / context.overflow and surfaces them
on turn.ended{ reason: 'failed' }, but the ACP layer only knew how
to map auth errors — every other failure was swallowed into end_turn
and clients had no way to tell the model service was unavailable.

Change

Add isProviderError() in packages/acp-server/src/events-map.ts
(sibling to isAuthError()) and a third branch in both
AcpSession.onTurnEnded and mapPromptLaunchError: provider /
context failures now reject with
RequestError.internalError({ code, message }, "model provider reported an error"). The engine's full text rides in the JSON-RPC
data payload so clients can log it; the wire message stays generic
to keep provider-supplied PII out of every error toast.

• Auth failures keep their existing auth_required mapping.
• Content-filter failures (provider.filtered) keep the legacy
  refusal mapping in turnEndReasonToStopReason and fall through
  to the fixed generic message at launch (where no turn exists yet to
  attach a refusal to).

The scripted provider helper gains mockNextProviderError(code, message)
so end-to-end tests can throw a coded provider error without going
through the network layer.

Test plan

• packages/acp-server/test/events-map.test.ts — new unit tests for
  isAuthError / isProviderError / turnEndReasonToStopReason,
  covering every code in each set plus the cross-set negatives
  (provider.filtered must not match isProviderError).
• packages/acp-server/test/e2e-turn.test.ts — mapPromptLaunchError
  unit tests for provider.api_error / context.overflow /
  provider.filtered (legacy) and a scripted end-to-end test that
  throws a coded provider error mid-turn and asserts the prompt
  rejects as internalError carrying the engine message in data.

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🦋 Changeset detected

Latest commit: 428e850

The changes in this PR will be included in the next version bump.

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@moonshot-ai/kimi-code	Patch

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