docs(how-to): Add guide for isolating async applications in tests

[AreteDriver]

Summary

Adds a new how-to guide demonstrating two common failure modes when testing async applications (e.g., FastAPI, Quart, aiohttp) with pytest-asyncio, and how to fix them using fixture-scoped imports and temporary directory overrides.

Motivation

Testing async web frameworks often involves importing an app object and overriding dependencies. Two subtle isolation traps repeatedly bite production codebases:

1. Stale app reference via module-level import — importlib.reload creates a new app instance, but other test modules hold stale references to the old one through module-level imports.
2. Default path leak — An app or service with a default file-system path in its constructor writes state that bleeds across tests.

pytest-asyncio's fixture scoping makes these easy to prevent, but the failure modes aren't well documented anywhere in the asyncio ecosystem.

Scope

• docs/how-to-guides/isolate_async_apps.rst — the guide
• docs/how-to-guides/isolate_async_apps_example.py — runnable examples
• docs/how-to-guides/index.rst — add entry to toctree

Verification

The example code passes on pytest-asyncio 0.24+ with FastAPI as the demo framework (any async web framework exhibits the same pattern). Both failure modes are reproduced with failing assertions, then fixed.

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Author's note: These patterns were extracted from production test suites (github.com/AreteDriver/animus and github.com/AreteDriver/BenchGoblins). Happy to adjust to match project conventions.