Intermittent attach_link failure (EBPF_INSUFFICIENT_BUFFER) with 8 threads when running sequential stress test cases

## Summary

When running two stress test cases (native + JIT) back-to-back with 8 threads and 2-minute duration, the **second** test case intermittently gets a single `hook.attach_link()` failure with error code 23 (`EBPF_INSUFFICIENT_BUFFER`).

## Reproduction

```
.\ebpf_stress_tests_um.exe -tt=8 -td=2
```

- Running either test case alone with 8 threads: **passes**
- Running both with 4 threads: **passes**
- Running both with 8 threads: **1 failure** in the second test case (~1 in 10,000 iterations)
- The failure occurs regardless of which test case runs second

## Root Cause (Preliminary)

The error propagates through the NMR provider attach path. Each stress thread creates a `single_instance_hook_t` (NMR provider) that lives for the thread's duration. With 8 threads rapidly registering/deregistering providers across ~5000 iterations each, some internal state from the first test case appears to leak into the second test case, causing a brief window where `attach_link()` returns `EBPF_INSUFFICIENT_BUFFER`.

The `~_single_instance_hook()` destructor properly calls `NmrDeregisterProvider` and waits for completion, so the leak may be deeper in the execution context or NMR layer.

## Impact

- This is a **pre-existing issue** that was previously hidden because `main()` in `ebpf_stress_tests.cpp` discarded the `session.run()` return value (always exited 0)
- Only affects stress tests with high thread counts running multiple test cases sequentially
- Does not affect production code

## Workaround

Running the native test case before JIT reduces (but does not eliminate) the failure rate. The test case order has been adjusted in PR #5072.

## Related

- PR #5072 (stress-um-jit-or-native) — splits JIT/native stress tests and fixes the exit code bug that was hiding this issue