fix(cli,session): remove nest_asyncio, make chat-profile config async-safe

[pidefrem]

Summary

Problem 1 — nest_asyncio breaks Python 3.14

nest_asyncio.apply() is called at import time in chainlit/cli/__init__.py.
nest_asyncio <= 1.6.0 internally calls asyncio.ensure_future(future, loop=self),
where the loop= keyword was removed in Python 3.14 (bpo-39529).
This corrupts asyncio task registration, causing asyncio.current_task() to return
None inside running coroutines.

The symptom is a white page on every Chainlit app running on Python 3.14:
starlette's FileResponse calls anyio.to_thread.run_sync(), which calls
sniffio.current_async_library(), which calls asyncio.current_task() -> None
-> anyio.NoEventLoopError -> HTTP 500 on every static asset.

Problem 2 — get_config() relied on nest_asyncio for re-entrant event loop

WebsocketSession.get_config() called asyncio.get_event_loop().run_until_complete()
from within the already-running event loop (the SocketIO connect handler).
This only worked because nest_asyncio patched run_until_complete to be re-entrant.
Removing nest_asyncio without fixing this would silently break chat-profile
config overrides on all Python versions (3.10-3.13), not just 3.14.

Fix

Commit 1 (b1a6bb4): Remove nest_asyncio entirely.
asyncio.run(start()) is a top-level entry point; it does not need re-entrant
loop support. Also removes the nest-asyncio dependency from pyproject.toml.
Adds a regression test to prevent silent reintroduction.

Commit 2 (55bf45b): Exclude .pytest_cache from Prettier checks.

Commit 3 (2406b99): Make chat-profile config resolution async-safe.

• get_config() now returns the cached config immediately (no run_until_complete)
• New async resolve_config() method properly awaits set_chat_profiles()
• connect() handler calls await session.resolve_config() after construction
• 9 new tests covering overrides, idempotency, error handling, unknown profiles,
  and a regression test verifying run_until_complete is never called
• Adds hatchling to the mypy optional deps (it ships py.typed;
  fixes a pre-existing import-not-found mypy error in build.py)

Testing

All 798 tests pass on Python 3.10, 3.11, 3.12, and 3.13.
Python 3.14 cannot be tested yet (pydantic-core lacks a 3.14 wheel).

Note: the requires-python < 3.14 upper bound is intentionally left unchanged.
Lifting it is a separate decision once the full dependency chain supports 3.14.

Refs #2767

---

Known pre-existing issue (out of scope)

cypress/support/run.ts:27 has a bug: access() from fs/promises returns a
Promise (always truthy), so the if (!access(entryPointPath)) check is dead code
and the file-existence guard never triggers. This predates this PR and should be
addressed separately.

[cubic-dev-ai]

No issues found across 4 files

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[cubic-dev-ai]

1 issue found across 3 files (changes from recent commits).

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[github-actions]

This PR is stale because it has been open for 14 days with no activity.

[pidefrem]

Not stale — this PR is active. I just pushed 012f46d1 (docstring clarification for resolve_config caching), and the branch is up to date with main with no conflicts. It fixes #2952.

It's been waiting on a maintainer review. Could someone take a look when they have a chance? Happy to rebase or address feedback. Thanks! 🙏