Resolve #6730

Add staggered timing between AWS tests, Kubernetes tests, and remote server tests.

• Kubernetes tests run on a local Kind cluster.
• Remote server tests require launching a local Docker image and connecting to it.

Both are resource-heavy on the Buildkite agent VM, increasing failure risk if run alongside other tests.

Before this PR

Fully sequential tests (e.g., waiting for AWS → Kubernetes → remote, totaling 2.5 hours):

|-----aws tests-----|(1 hour)  
                        |-----kubernetes tests-----|(2 hours total, 1 hour runtime)  
                                                                  |-----remote server tests-----|(2.5 hours total, 0.5 hour runtime)  

After this PR:

Tests overlap intelligently to reduce total time and resource contention:

|-------------aws tests-------------|(1 hour)  
    (25m delay)|--------kubernetes tests--------|(1.4 hours total, 1 hour runtime)  
          (35m delay)|--remote server tests----|(1.1 hour total, 0.5 hour runtime)  

Key changes:

1. Kubernetes tests start after 25m (0.4-hour delay), when most AWS tests finish.
2. Remote tests start after 35m, further reducing VM load.
3. Total runtime drops to ~1.4 hours (vs. 2.5 hours sequential) with lower failure rates.

Tested on my personal repo

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)