Check version incompatibility (#738)

When we bump up `major.minor` we want to make sure clients can
immediately detect a version change and appropriately error out. It is
not reasonable to keep checking for API-level backwards compatibility
across such version bumps. Over time, we will make the check based only
on the major version perhaps.

### Test Plan

Manually updated `__version__` in the client SDK to be "0.1.0" which is
incompatible with server's current version "0.0.63", got the following
error:

<img width="1077" alt="image"
src="https://github.com/user-attachments/assets/06ae4659-0a25-4c4c-a999-ce44678d4e6f"
/>

Without this update, the CLI worked correctly.

llama_stack/distribution/server/server.py

@@ -16,6 +16,8 @@ import traceback
 import warnings
 
 from contextlib import asynccontextmanager
+
+from importlib.metadata import version as parse_version
 from pathlib import Path
 from typing import Any, Union
 
@@ -228,6 +230,52 @@ class TracingMiddleware:
             await end_trace()
 
 
+class ClientVersionMiddleware:
+    def __init__(self, app):
+        self.app = app
+        self.server_version = parse_version("llama-stack")
+
+    async def __call__(self, scope, receive, send):
+        if scope["type"] == "http":
+            headers = dict(scope.get("headers", []))
+            client_version = headers.get(b"x-llamastack-client-version", b"").decode()
+            if client_version:
+                try:
+                    client_version_parts = tuple(
+                        map(int, client_version.split(".")[:2])
+                    )
+                    server_version_parts = tuple(
+                        map(int, self.server_version.split(".")[:2])
+                    )
+                    if client_version_parts != server_version_parts:
+
+                        async def send_version_error(send):
+                            await send(
+                                {
+                                    "type": "http.response.start",
+                                    "status": 426,
+                                    "headers": [[b"content-type", b"application/json"]],
+                                }
+                            )
+                            error_msg = json.dumps(
+                                {
+                                    "error": {
+                                        "message": f"Client version {client_version} is not compatible with server version {self.server_version}. Please upgrade your client."
+                                    }
+                                }
+                            ).encode()
+                            await send(
+                                {"type": "http.response.body", "body": error_msg}
+                            )
+
+                        return await send_version_error(send)
+                except (ValueError, IndexError):
+                    # If version parsing fails, let the request through
+                    pass
+
+        return await self.app(scope, receive, send)
+
+
 def main():
     """Start the LlamaStack server."""
     parser = argparse.ArgumentParser(description="Start the LlamaStack server.")
@@ -291,6 +339,7 @@ def main():
 
     app = FastAPI(lifespan=lifespan)
     app.add_middleware(TracingMiddleware)
+    app.add_middleware(ClientVersionMiddleware)
 
     try:
         impls = asyncio.run(construct_stack(config))