refactor(cliproxy): remove orphaned bg keepalive wiring (#1353)

Red-team review of #1340 found that backgroundProbe / shouldKeepSessionProxiesAlive /
startKeepAliveWatcher and related types were never reachable: the sole caller
(claude-launcher.ts) passed no hasBackgroundWorkerUsingBaseUrl detector, so
backgroundProbe was always undefined and the keepalive branch never executed.

Remove the unreachable scaffold:
- Delete CLAUDE_BG_WORKER_ARGS, hasClaudeBackgroundWorkerUsingBaseUrl{,InProcessList},
  shouldKeepSessionProxiesAlive, ShouldKeepSessionProxiesAliveOptions, and the
  SessionProxyKeepAliveOptions interface from session-bridge.ts
- Remove the backgroundProbe/startKeepAliveWatcher block and keepalive exit branch
  from setupCleanupHandlers; the function now always calls stopSessionResources on
  Claude exit
- Remove backgroundKeepAliveBaseUrl wiring from claude-launcher.ts
- Remove the execFileSync import (was only used by the deleted ps-based detector)
- Update test file: remove tests for deleted exports, keep placeholder

A trusted bg-worker detector can be re-added in a future PR if the feature is
actually needed; the keepalive logic in shouldKeepSessionProxiesAlive can be
restored then.

tests/unit/cliproxy/session-bridge-bg-keepalive.test.ts

@@ -1,57 +1,17 @@
-import { describe, it, expect } from 'bun:test';
-import {
-  hasClaudeBackgroundWorkerUsingBaseUrlInProcessList,
-  shouldKeepSessionProxiesAlive,
-} from '../../../src/cliproxy/executor/session-bridge';
+/**
+ * The bg-keepalive feature (shouldKeepSessionProxiesAlive, hasClaudeBackgroundWorker*,
+ * backgroundProbe) was removed as dead code: the sole caller (claude-launcher.ts) never
+ * provided a trusted hasBackgroundWorkerUsingBaseUrl detector, so the keepalive path was
+ * unreachable. The ps-based detector was also spoofable (fixed in #1340). This test file
+ * is kept as a placeholder to prevent the test suite from failing on missing imports.
+ *
+ * If a trusted bg-worker detector is implemented in the future, new tests belong here.
+ */
+
+import { describe, it } from 'bun:test';
 
 describe('session bridge background proxy keepalive', () => {
-  it('keeps per-session proxies alive after a clean Claude exit when a bg worker inherited the base URL', () => {
-    expect(
-      shouldKeepSessionProxiesAlive({
-        code: 0,
-        signal: null,
-        backgroundKeepAliveBaseUrl: 'http://127.0.0.1:64314/api/provider/codex',
-        hasBackgroundWorkerUsingBaseUrl: () => true,
-      })
-    ).toBe(true);
-  });
-
-  it('cleans up per-session proxies when no bg worker inherited the base URL', () => {
-    expect(
-      shouldKeepSessionProxiesAlive({
-        code: 0,
-        signal: null,
-        backgroundKeepAliveBaseUrl: 'http://127.0.0.1:64314/api/provider/codex',
-        hasBackgroundWorkerUsingBaseUrl: () => false,
-      })
-    ).toBe(false);
-  });
-
-  it('cleans up per-session proxies for signaled Claude exits', () => {
-    expect(
-      shouldKeepSessionProxiesAlive({
-        code: null,
-        signal: 'SIGTERM',
-        backgroundKeepAliveBaseUrl: 'http://127.0.0.1:64314/api/provider/codex',
-        hasBackgroundWorkerUsingBaseUrl: () => true,
-      })
-    ).toBe(false);
-  });
-
-  it('detects Claude bg workers that inherited the exact base URL', () => {
-    const baseUrl = 'http://127.0.0.1:64314/api/provider/codex';
-    const processList = `
-123 /opt/claude/claude.exe --bg-spare /tmp/claim.sock ANTHROPIC_BASE_URL=${baseUrl} ANTHROPIC_AUTH_TOKEN=ccs-internal-managed
-456 /opt/claude/claude.exe --bg-spare /tmp/claim.sock ANTHROPIC_BASE_URL=http://127.0.0.1:62075/api/provider/codex
-789 /opt/claude/claude.exe --print ANTHROPIC_BASE_URL=${baseUrl}
-`;
-
-    expect(hasClaudeBackgroundWorkerUsingBaseUrlInProcessList(processList, baseUrl)).toBe(true);
-    expect(
-      hasClaudeBackgroundWorkerUsingBaseUrlInProcessList(
-        processList,
-        'http://127.0.0.1:65535/api/provider/codex'
-      )
-    ).toBe(false);
+  it('bg keepalive wiring removed — no dead code to test', () => {
+    // No-op: feature scaffold removed per red-team finding on #1340.
   });
 });