test: repair route harness error rendering + stop AI-checker spawning real processes in tests (#97)

* fix(test): share route error handler with test harness + fix stale assertions

The route test harness built a bare Fastify instance without the production
global error handler (server.ts), so structured errors thrown by route helpers
(findSessionOrFail → 404, parseBody → 400) fell through to Fastify's default
handler — yielding a `{statusCode,error,message}` body instead of the
`{success:false,...}` shape, and the tests asserted the old implicit-200
behavior. 51 route tests across 7 files were red.

- Extract the handler into src/web/route-error-handler.ts; server.ts and the
  test harness now install the identical handler (single source of truth).
- Correct stale assertions across route test files: throw-based error paths
  now assert 404 (unknown session) / 400 (invalid body); genuine in-handler
  `return createErrorResponse(...)` paths (200 + success:false) left untouched.
- Reformat a few test files prettier flagged (pre-existing non-compliance).

Route suite: 307/307 passing (was 256/307). No production behavior change.

* test(respawn): mock child_process so AI checker never spawns real processes

respawn-controller.test.ts drives the AI idle checker (ai-checker-base), whose
runCheck() spawns a real `tmux new-session` running `claude -p`. The AI-enabled
tests only assert the ai_checking state transition (then cancel/stop), so the
spawn produced stray real tmux sessions and claude processes on every run — the
reason `npm test` (full suite) was unsafe to run inside a managed session.

Mock node:child_process here (mirroring ai-idle-checker.test.ts), spreading the
real module so `exec` stays intact for transitively-imported modules
(tmux-manager calls promisify(exec) at load). With this, the full non-mobile
suite runs without spawning any real tmux/claude.

---------

Co-authored-by: Teigen <teigen@TeigendeMac-mini.local>

test/routes/session-routes.test.ts

@@ -103,7 +103,7 @@ describe('session-routes', () => {
         method: 'GET',
         url: '/api/sessions/nonexistent',
       });
-      expect(res.statusCode).toBe(200); // returns error in body, not HTTP 404
+      expect(res.statusCode).toBe(404);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
       expect(body.error).toBeDefined();
@@ -174,7 +174,7 @@ describe('session-routes', () => {
         url: '/api/sessions/nonexistent/name',
         payload: { name: 'test' },
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(404);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -227,7 +227,7 @@ describe('session-routes', () => {
         url: '/api/sessions/nonexistent/input',
         payload: { input: 'hello' },
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(404);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -238,7 +238,7 @@ describe('session-routes', () => {
         url: `/api/sessions/${harness.ctx._sessionId}/input`,
         payload: {},
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(400);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -265,7 +265,7 @@ describe('session-routes', () => {
         url: `/api/sessions/${harness.ctx._sessionId}/resize`,
         payload: { cols: 501, rows: 24 },
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(400);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -276,7 +276,7 @@ describe('session-routes', () => {
         url: `/api/sessions/${harness.ctx._sessionId}/resize`,
         payload: { cols: 80, rows: 201 },
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(400);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -287,7 +287,7 @@ describe('session-routes', () => {
         url: `/api/sessions/${harness.ctx._sessionId}/resize`,
         payload: { cols: 0, rows: 24 },
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(400);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -312,7 +312,7 @@ describe('session-routes', () => {
         method: 'GET',
         url: '/api/sessions/nonexistent/terminal',
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(404);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -338,7 +338,7 @@ describe('session-routes', () => {
         url: `/api/sessions/${harness.ctx._sessionId}/run`,
         payload: { prompt: '' },
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(400);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -349,7 +349,7 @@ describe('session-routes', () => {
         url: '/api/sessions/nonexistent/run',
         payload: { prompt: 'test' },
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(404);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -386,7 +386,7 @@ describe('session-routes', () => {
         method: 'POST',
         url: '/api/sessions/nonexistent/interactive',
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(404);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -450,7 +450,7 @@ describe('session-routes', () => {
         method: 'GET',
         url: '/api/sessions/nonexistent/output',
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(404);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
@@ -586,7 +586,7 @@ describe('session-routes', () => {
           resumeSessionId: 'not-a-uuid',
         },
       });
-      expect(res.statusCode).toBe(200);
+      expect(res.statusCode).toBe(400);
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });