test: speed up slow tests (backoff + subprocess + IMDS network) (#11797)

Cuts shard-3 local runtime in half by neutralizing real wall-clock
waits across three classes of slow test:

## 1. Retry backoff mocks

- tests/run_agent/conftest.py (NEW): autouse fixture mocks
  jittered_backoff to 0.0 so the `while time.time() < sleep_end`
  busy-loop exits immediately. No global time.sleep mock (would
  break threading tests).
- test_anthropic_error_handling, test_413_compression,
  test_run_agent_codex_responses, test_fallback_model: per-file
  fixtures mock time.sleep / asyncio.sleep for retry / compression
  paths.
- test_retaindb_plugin: cap the retaindb module's bound time.sleep
  to 0.05s via a per-test shim (background writer-thread retries
  sleep 2s after errors; tests don't care about exact duration).
  Plus replace arbitrary time.sleep(N) waits with short polling
  loops bounded by deadline.

## 2. Subprocess sleeps in production code

- test_update_gateway_restart: mock time.sleep. Production code
  does time.sleep(3) after `systemctl restart` to verify the
  service survived. Tests mock subprocess.run \u2014 nothing actually
  restarts \u2014 so the wait is dead time.

## 3. Network / IMDS timeouts (biggest single win)

- tests/conftest.py: add AWS_EC2_METADATA_DISABLED=true plus
  AWS_METADATA_SERVICE_TIMEOUT=1 and ATTEMPTS=1. boto3 falls back
  to IMDS (169.254.169.254) when no AWS creds are set. Any test
  hitting has_aws_credentials() / resolve_aws_auth_env_var() (e.g.
  test_status, test_setup_copilot_acp, anything that touches
  provider auto-detect) burned ~2-4s waiting for that to time out.
- test_exit_cleanup_interrupt: explicitly mock
  resolve_runtime_provider which was doing real network auto-detect
  (~4s). Tests don't care about provider resolution \u2014 the agent
  is already mocked.
- test_timezone: collapse the 3-test "TZ env in subprocess" suite
  into 2 tests by checking both injection AND no-leak in the same
  subprocess spawn (was 3 \u00d7 3.2s, now 2 \u00d7 4s).

## Validation

| Test | Before | After |
|---|---|---|
| test_anthropic_error_handling (8 tests) | ~80s | ~15s |
| test_413_compression (14 tests) | ~18s | 2.3s |
| test_retaindb_plugin (67 tests) | ~13s | 1.3s |
| test_status_includes_tavily_key | 4.0s | 0.05s |
| test_setup_copilot_acp_skips_same_provider_pool_step | 8.0s | 0.26s |
| test_update_gateway_restart (5 tests) | ~18s total | ~0.35s total |
| test_exit_cleanup_interrupt (2 tests) | 8s | 1.5s |
| **Matrix shard 3 local** | **108s** | **50s** |

No behavioral contract changed \u2014 tests still verify retry happens,
service restart logic runs, etc.; they just don't burn real seconds
waiting for it.

Supersedes PR #11779 (those changes are included here).

tests/run_agent/test_413_compression.py

@@ -19,6 +19,24 @@ import pytest
 
 from agent.context_compressor import SUMMARY_PREFIX
 from run_agent import AIAgent
+import run_agent
+
+
+# ---------------------------------------------------------------------------
+# Fast backoff for compression retry tests
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture(autouse=True)
+def _no_compression_sleep(monkeypatch):
+    """Short-circuit the 2s time.sleep between compression retries.
+
+    Production code has ``time.sleep(2)`` in multiple places after a 413/context
+    compression, for rate-limit smoothing. Tests assert behavior, not timing.
+    """
+    import time as _time
+    monkeypatch.setattr(_time, "sleep", lambda *_a, **_k: None)
+    monkeypatch.setattr(run_agent, "jittered_backoff", lambda *a, **k: 0.0)
 
 
 # ---------------------------------------------------------------------------