Fix protobuf body parsing causing 400 errors on agent heartbeat

Plug.Parsers was trying to parse protobuf bodies as JSON, failing with
400 errors before the request reached the controller.

Changes:
- Added custom BodyReader to endpoint that skips parsing for protobuf
- When Content-Type is application/x-protobuf, return empty body to parser
- Controller reads the raw body directly for protobuf requests
- Added error handling for protobuf decode failures in heartbeat endpoint

This fixes the 400 errors agents were seeing on heartbeat requests.

lib/towerops_web/controllers/api/agent_controller.ex

@@ -98,14 +98,31 @@ defmodule ToweropsWeb.Api.AgentController do
       case get_req_header(conn, "content-type") do
         ["application/x-protobuf" | _] ->
           # Decode protobuf
-          {:ok, body, _conn} = Plug.Conn.read_body(conn)
-          proto_metadata = HeartbeatMetadata.decode(body)
+          case Plug.Conn.read_body(conn) do
+            {:ok, body, _conn} ->
+              try do
+                proto_metadata = HeartbeatMetadata.decode(body)
 
-          %{
-            "version" => proto_metadata.version,
-            "hostname" => proto_metadata.hostname,
-            "uptime_seconds" => proto_metadata.uptime_seconds
-          }
+                %{
+                  "version" => proto_metadata.version,
+                  "hostname" => proto_metadata.hostname,
+                  "uptime_seconds" => proto_metadata.uptime_seconds
+                }
+              rescue
+                e ->
+                  require Logger
+
+                  Logger.error("Failed to decode protobuf heartbeat: #{inspect(e)}")
+                  # Return empty metadata on decode error
+                  %{}
+              end
+
+            {:error, reason} ->
+              require Logger
+
+              Logger.error("Failed to read heartbeat body: #{inspect(reason)}")
+              %{}
+          end
 
         _ ->
           # JSON format (fallback)

lib/towerops_web/endpoint.ex

@@ -49,8 +49,25 @@ defmodule ToweropsWeb.Endpoint do
   plug Plug.Parsers,
     parsers: [:urlencoded, :multipart, :json],
     pass: ["*/*"],
+    body_reader: {ToweropsWeb.Endpoint.BodyReader, :read_body, []},
     json_decoder: Phoenix.json_library()
 
+  # Custom body reader that skips parsing for protobuf content type
+  defmodule BodyReader do
+    @moduledoc false
+    def read_body(conn, opts) do
+      case Plug.Conn.get_req_header(conn, "content-type") do
+        ["application/x-protobuf" | _] ->
+          # Don't parse protobuf, let the controller handle it
+          {:ok, "", conn}
+
+        _ ->
+          # Use default body reader for other content types
+          Plug.Conn.read_body(conn, opts)
+      end
+    end
+  end
+
   plug Plug.MethodOverride
   plug Plug.Head
   plug Plug.Session, @session_options