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.

apps/server/src/services/__tests__/stream-error-classifier.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest';
+import { classifyStreamError } from '../inference/stream-error-classifier.js';
+
+describe('classifyStreamError', () => {
+  it("classifies AbortError as 'stall'", () => {
+    const err = new Error('aborted');
+    err.name = 'AbortError';
+    expect(classifyStreamError(err)).toBe('stall');
+  });
+
+  it("classifies a 503 HTTP error as 'transient'", () => {
+    const err = Object.assign(new Error('Service Unavailable'), { status: 503 });
+    expect(classifyStreamError(err)).toBe('transient');
+  });
+
+  it("classifies a 500 HTTP error as 'transient'", () => {
+    const err = Object.assign(new Error('Internal Server Error'), { status: 500 });
+    expect(classifyStreamError(err)).toBe('transient');
+  });
+
+  it("classifies a 4xx HTTP error as 'non-retryable'", () => {
+    const err = Object.assign(new Error('Bad Request'), { status: 400 });
+    expect(classifyStreamError(err)).toBe('non-retryable');
+  });
+
+  it("classifies a generic Error as 'non-retryable'", () => {
+    expect(classifyStreamError(new Error('something went wrong'))).toBe('non-retryable');
+  });
+});