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

fix(cli): 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 and its mypy override.
Adds a regression test to prevent silent reintroduction.

fix(session): 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)

test(e2e): harden Chainlit process cleanup and Cypress CI.

• Kills the entire Chainlit process group (uv -> chainlit -> uvicorn)
  between specs and polls until the port is free, resolving an EADDRINUSE race
• Excludes .pytest_cache from Prettier checks
• Ensures the Cypress binary is present before tests run (a restored-but-incomplete
  cache on windows-latest left the npm package installed while the binary was
  missing) and sets fail-fast: false so one shard's flake no longer cancels the
  rest of the matrix

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

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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! 🙏

[dokterbob]

Thanks! Some much welcomed cleanup/fixup.

[github-actions]

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