feat: post-review backlog hardening (cancel/parser/stall/history/9502)

Five independent items from the post-review backlog. F1: Stop on an external
agent task now aborts the running child via a per-task AbortController registry
reachable from the cancel route, and finalizes the assistant message as
cancelled (fixing two latent bugs — catch blocks left the message streaming,
and warm success-paths wrote complete on an aborted turn); warm pools/worktrees
are preserved and the native path is unchanged. F2/F3: prune the tool-call
parser to its two load-bearing exports (unexport eight zero-caller symbols, add
a gate test for the <invoke>-as-text fallback) and route placeholder-rejection
logging through pino. F6: a 90s per-chunk stall-timeout wraps native inference's
fullStream via AbortSignal.any so a hung stream finalizes the message instead of
hanging — no retry (a pure classifyStreamError helper is added). F7: a read-only
view_session_history MCP tool (newest-N, chronological). F9: retire the unused
apps/coder/web :9502 fallback SPA, keeping every API/WS/health/MCP route.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/services/__tests__/cancel-registry.test.ts

@@ -0,0 +1,51 @@
+import { describe, it, expect } from 'vitest';
+import { createCancelRegistry } from '../cancel-registry.js';
+
+/**
+ * F1 — per-task abort wiring. The registry is the missing link between the Stop
+ * route and the in-flight external run: register an AbortController per task id,
+ * cancel(taskId) aborts its signal, the run's .finally deletes it. Pure (no DB /
+ * child / IO) so the abort + idempotency contract is unit-testable in isolation.
+ */
+describe('CancelRegistry (F1 abort wiring)', () => {
+  it('register hands back a fresh controller; cancel aborts its signal', () => {
+    const reg = createCancelRegistry();
+    const ac = reg.register('t1');
+    expect(ac.signal.aborted).toBe(false);
+    expect(reg.has('t1')).toBe(true);
+
+    expect(reg.cancel('t1')).toBe(true);
+    expect(ac.signal.aborted).toBe(true);
+  });
+
+  it('cancel on an unknown task returns false (native task / cancel-before-register)', () => {
+    const reg = createCancelRegistry();
+    expect(reg.has('nope')).toBe(false);
+    expect(reg.cancel('nope')).toBe(false);
+  });
+
+  it('double-Stop is idempotent: a second cancel never throws and the signal stays aborted', () => {
+    const reg = createCancelRegistry();
+    const ac = reg.register('t1');
+
+    expect(reg.cancel('t1')).toBe(true);
+    // The run-function has not hit its .finally yet, so the entry is still
+    // present — a rapid second Stop re-aborts (abort() no-ops) without throwing.
+    expect(() => reg.cancel('t1')).not.toThrow();
+    expect(reg.cancel('t1')).toBe(true);
+    expect(ac.signal.aborted).toBe(true);
+  });
+
+  it('cancel after delete returns false (cancel-after-natural-exit is safe)', () => {
+    const reg = createCancelRegistry();
+    reg.register('t1');
+    reg.delete('t1');
+    expect(reg.has('t1')).toBe(false);
+    expect(reg.cancel('t1')).toBe(false);
+  });
+
+  it('delete of an unknown id is a no-op (never throws)', () => {
+    const reg = createCancelRegistry();
+    expect(() => reg.delete('ghost')).not.toThrow();
+  });
+});