fix(payg): fire the usage-limit modal when an AI agent run hits the limit (#6638)

## Problem

We're getting 402s when an AI **agent** (chat) run hits the free
allowance / spending cap, but the frontend handles them poorly and never
pops the usage-limit modal.

The agent runs its tool calls **server-side** (loopback HTTP via
`PolicyExecutor`), so the 402 never reaches the `apiClient` interceptor
that pops the modal for direct calls. It was caught by the generic
tool-failure handler and flattened into a `CANNOT_CONTINUE` reason
string (`"The /api/v1/… tool failed: 402…"`), streamed as a `result`
event, and rendered as a scary chat bubble. This is the same gap the
policy auto-run path bridges (#6626) — one layer up.

## Fix

**Backend** (`proprietary`)
- `AiWorkflowResponse` gains `errorCode` + `errorSubscribed`.
- `AiWorkflowService` detects a downstream 401/402 entitlement sentinel
in its three tool-exec catch sites (`onToolCall`, `runPlan`,
`onConvertMarkdown`) and surfaces the structured code (+ `subscribed`)
on the terminal response instead of the raw failure text.
- Factored the 401/402 body extraction `PolicyEngine` already had into a
shared `DownstreamEntitlementError` util so the two server-side paths
can't drift.

**Frontend**
- New `usageLimitBridge` (`PAYG_LIMIT_REACHED_EVENT` +
`dispatchPaygLimitReached`) generalises the previously policy-only
bridge. Proprietary can't import the saas modal API (layering), so
server-side limit hits broadcast a window event the saas
`UsageLimitModalHost` opens the modal from. Migrated the policy path
onto it.
- `ChatContext` fires the matching modal (free → subscribe, subscribed →
raise cap) on the limit result **and** on a direct 402, replacing the
raw reason with a brief friendly line
(`chat.responses.usage_limit_reached`).

No Python engine changes — the charge/402 happens on the Java tool
endpoint that Java itself calls.

## Test plan

- [x] `:proprietary:compileJava` + `spotlessCheck` clean
- [x] `AiWorkflowServiceTest` + `PolicyEngineTest` green
- [x] eslint, proprietary + saas typechecks clean
- [ ] Manual: drive an agent run over the limit → brief line in chat +
the right modal (free vs cap)

> Note: proprietary test compilation is currently blocked on the
pre-existing `InitialSecuritySetupTest` 6-arg ctor break (unrelated,
tracked separately); verified locally by temporarily patching it.

frontend/editor/src/proprietary/components/policies/usePolicyAutoRun.ts

@@ -27,9 +27,8 @@ import {
 import type {
   PolicyRunStatus,
   PolicyRunView,
-  PolicyLimitReachedDetail,
 } from "@app/services/policyPipeline";
-import { POLICY_LIMIT_REACHED_EVENT } from "@app/services/policyPipeline";
+import { dispatchPaygLimitReached } from "@app/services/usageLimitBridge";
 import type { FileId } from "@app/types/file";
 import { createStirlingFilesAndStubs } from "@app/services/fileStubHelpers";
 import type { StirlingFile, StirlingFileStub } from "@app/types/fileContext";
@@ -87,15 +86,7 @@ export function usePolicyAutoRun(): void {
     if (code !== "PAYG_LIMIT_REACHED" && code !== "FEATURE_DEGRADED") return;
     if (firedLimitModal.current.has(view.runId)) return;
     firedLimitModal.current.add(view.runId);
-    try {
-      window.dispatchEvent(
-        new CustomEvent<PolicyLimitReachedDetail>(POLICY_LIMIT_REACHED_EVENT, {
-          detail: { subscribed: view.errorSubscribed ?? null },
-        }),
-      );
-    } catch {
-      // non-browser env (tests / SSR) — no-op.
-    }
+    dispatchPaygLimitReached(view.errorSubscribed ?? null);
   }, []);
 
   // Dispatch: for each active policy × each session file not yet run, fire a run.