Fixes issue #2068 (#2083)

## Motivation

To fix https://github.com/exo-explore/exo/issues/2068

## Changes

Adds queue shutdown logic & hard-timeouts for closing server.

## Why It Works

Prevents API from hanging more than 5 seconds.

src/exo/api/main.py

@@ -20,6 +20,7 @@ from fastapi.staticfiles import StaticFiles
 from hypercorn.asyncio import serve  # pyright: ignore[reportUnknownVariableType]
 from hypercorn.config import Config
 from hypercorn.typing import ASGIFramework
+from hypercorn.utils import LifespanTimeoutError
 from loguru import logger
 
 from exo.api.adapters.chat_completions import (
@@ -1856,12 +1857,21 @@ class API:
                     await anyio.sleep_forever()
                 finally:
                     with anyio.CancelScope(shield=True):
+                        # IMPORTANT: when new queues are added, update this (for proper shutdown semantics)
+                        self._shutdown_queues(self._text_generation_queues)
+                        self._shutdown_queues(self._image_generation_queues)
+
                         shutdown_ev.set()
         finally:
             self._event_log.close()
             self.command_sender.close()
             self.event_receiver.close()
 
+    @staticmethod
+    def _shutdown_queues[K, V](queues: dict[K, Sender[V]]):
+        for v in queues.values():
+            v.close()
+
     async def run_api(self, ev: anyio.Event):
         cfg = Config()
         cfg.bind = [f"0.0.0.0:{self.port}"]
@@ -1869,12 +1879,23 @@ class API:
         cfg.accesslog = None
         cfg.errorlog = "-"
         cfg.logger_class = InterceptLogger
+
+        # prevents hangs when mid-request and connection refuses to close
+        cfg.graceful_timeout = 2  # seconds
+        cfg.shutdown_timeout = 3  # seconds
+
         with anyio.CancelScope(shield=True):
-            await serve(
-                cast(ASGIFramework, self.app),
-                cfg,
-                shutdown_trigger=ev.wait,
-            )
+            try:
+                await serve(
+                    cast(ASGIFramework, self.app),
+                    cfg,
+                    shutdown_trigger=ev.wait,
+                )
+            except LifespanTimeoutError as e:
+                logger.warning(
+                    "Graceful server shutdown timed out, some connections forcebly closed"
+                )
+                logger.opt(exception=e).debug("")
 
     async def _apply_state(self):
         with self.event_receiver as events: