Kill Uvicorn server via gc.get_objects() in ui.run_with (zauberzeug#5686)

### Motivation

This PR fixes zauberzeug#3253, how `app.shutdown` does not work when we do
`ui.run_with` as `Server.instance` was never set.

### Analysis

Note that upstream is extremely unlikely to be able to supply it to us:

- FastAPI CLI simply calls `uvicorn.run`
https://github.com/fastapi/fastapi-cli/blob/e62b61d6c56b77d39310a1c49e8eb7abdad2785a/src/fastapi_cli/cli.py#L218-L233
- `uvicorn.run` doesn't even return `server`
https://github.com/Kludex/uvicorn/blob/918dae6ef91917a5824ab48c46026e07b7c80beb/uvicorn/main.py#L579-L602

### Implementation (Copilot)

**Improvements to shutdown handling:**

* Updated the `shutdown` method in `nicegui/app/app.py` to use garbage
collection to find and set the `should_exit` flag on the correct
`uvicorn.Server` instance if the standard reference is unavailable,
increasing reliability of server shutdown.
* Added the `gc` and `uvicorn` imports to support the improved shutdown
logic in `nicegui/app/app.py`.
[[1]](diffhunk://#diff-adae3337464d5c5adf411c1b2e4b52034747030db2de3f87cfc83fea39763f96R2)
[[2]](diffhunk://#diff-adae3337464d5c5adf411c1b2e4b52034747030db2de3f87cfc83fea39763f96R13)
* **Human note: Server instance is always unavailable when using
`ui.run_with`, and always available when using `ui.run`**

**Application instance management:**

* Introduced a new global variable `fastapi_app` in `nicegui/core.py` to
hold a reference to the FastAPI application, and ensured it is set
during application startup in `nicegui/ui_run_with.py`.
[[1]](diffhunk://#diff-5ed898650c8cdc47c39575a2e752cfe07edba4c755c1c2390460373640983081R6)
[[2]](diffhunk://#diff-5ed898650c8cdc47c39575a2e752cfe07edba4c755c1c2390460373640983081R15)
[[3]](diffhunk://#diff-8af9e19f881b73ad9d426d4e341acfbc54b5f68e57e68d8eb2e7866fdde76f24R95-R96)
* **Human note: Prevents killing the wrong server, in case there are
multiple!**

### Progress

- [x] I chose a meaningful title that completes the sentence: "If
applied, this PR will..."
- [x] The implementation is complete.
- [x] If this PR addresses a security issue, it has been coordinated via
the [security
advisory](https://github.com/zauberzeug/nicegui/security/advisories/new)
process.
- [x] Pytests is out-of-scope.
- [x] Documentation is not necessary for a capability expansion of an
already-there function.

---------

Co-authored-by: Falko Schindler <falko@zauberzeug.com>

Author: evnchn

nicegui/app/app.py

@@ -189,7 +189,7 @@ def shutdown(self) -> None:
         """
         if self.native.main_window:
             self.native.main_window.destroy()
-        if self.config.reload:
+        if self.config.reload or Server.instance.config.should_reload:
             os.kill(os.getppid(), getattr(signal, 'CTRL_C_EVENT' if platform.system() == 'Windows' else 'SIGINT'))
         else:
             Server.instance.should_exit = True

nicegui/server.py

@@ -18,7 +18,7 @@ class CustomServerConfig(uvicorn.Config):
 
 
 class Server(uvicorn.Server):
-    instance: Server
+    instance: uvicorn.Server
 
     @classmethod
     def create_singleton(cls, config: CustomServerConfig) -> None:

nicegui/ui_run_with.py

@@ -1,7 +1,9 @@
+import gc
 from contextlib import asynccontextmanager
 from pathlib import Path
 from typing import Any, Callable, Literal, Optional, Union
 
+import uvicorn
 from fastapi import FastAPI
 from fastapi.middleware.gzip import GZipMiddleware
 from starlette.types import ASGIApp
@@ -11,6 +13,7 @@
 from .language import Language
 from .middlewares import RedirectWithPrefixMiddleware, SetCacheControlMiddleware
 from .nicegui import _shutdown, _startup
+from .server import Server
 
 
 def run_with(
@@ -86,6 +89,16 @@ def run_with(
 
     @asynccontextmanager
     async def lifespan_wrapper(app):
+        def _get_server_instance() -> Optional[uvicorn.Server]:
+            for server in (obj for obj in gc.get_objects() if isinstance(obj, uvicorn.Server)):
+                wrapped = server.config.loaded_app
+                while wrapped is not None:
+                    if wrapped is app:
+                        return server
+                    wrapped = getattr(wrapped, 'app', None)
+            return None
+        if (instance := _get_server_instance()) is not None:
+            Server.instance = instance
         await _startup()
         async with main_app_lifespan(app) as state:
             yield state