Add streaming exec and file transfer endpoints

Add WebSocket-based streaming exec endpoint and streaming file
upload/download endpoints to the control plane API. Includes new
host agent RPC methods (ExecStream, StreamWriteFile, StreamReadFile),
envd client streaming support, and OpenAPI spec updates.

internal/api/middleware.go

@@ -1,8 +1,11 @@
 package api
 
 import (
+	"bufio"
 	"encoding/json"
+	"fmt"
 	"log/slog"
+	"net"
 	"net/http"
 	"time"
 
@@ -61,6 +64,10 @@ func requestLogger() func(http.Handler) http.Handler {
 	}
 }
 
+func decodeJSON(r *http.Request, v any) error {
+	return json.NewDecoder(r.Body).Decode(v)
+}
+
 type statusWriter struct {
 	http.ResponseWriter
 	status int
@@ -70,3 +77,18 @@ func (w *statusWriter) WriteHeader(status int) {
 	w.status = status
 	w.ResponseWriter.WriteHeader(status)
 }
+
+// Hijack implements http.Hijacker, required for WebSocket upgrade.
+func (w *statusWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	if hj, ok := w.ResponseWriter.(http.Hijacker); ok {
+		return hj.Hijack()
+	}
+	return nil, nil, fmt.Errorf("underlying ResponseWriter does not implement http.Hijacker")
+}
+
+// Flush implements http.Flusher, required for streaming responses.
+func (w *statusWriter) Flush() {
+	if fl, ok := w.ResponseWriter.(http.Flusher); ok {
+		fl.Flush()
+	}
+}