[proxy] LLM parsers, pricing, and builtin middlewares (OpenAI, Anthropic, Vertex AI, AWS Bedrock)

Request/response parsers and SSE/event-stream metering, the embedded pricing
table, and the builtin middleware set: request parser, router, policy
limit-check/record, cost meter, guardrail, identity inject, response parser.
Includes the path-routed providers — Google Vertex AI (keyfile:: service-account
OAuth minting) and AWS Bedrock (bearer auth, invoke/converse/streaming, optional
/bedrock prefix) — plus the Models allowlist and unmeterable-publisher deny.

proxy/internal/middleware/builtin/llm_response_parser/factory.go

@@ -0,0 +1,43 @@
+package llm_response_parser
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+
+	"github.com/netbirdio/netbird/proxy/internal/middleware"
+	"github.com/netbirdio/netbird/proxy/internal/middleware/builtin"
+)
+
+// Factory constructs configured Middleware instances for the registry.
+type Factory struct{}
+
+// ID returns the registry identifier.
+func (Factory) ID() string { return ID }
+
+// New decodes RawConfig (empty / null / "{}" all accepted) and returns
+// a configured Middleware. Construction never fails on a well-formed
+// empty config; only structurally invalid JSON is rejected.
+func (Factory) New(rawConfig []byte) (middleware.Middleware, error) {
+	cfg, err := decodeConfig(rawConfig)
+	if err != nil {
+		return nil, fmt.Errorf("decode config: %w", err)
+	}
+	return New(cfg), nil
+}
+
+func decodeConfig(raw []byte) (config, error) {
+	trimmed := bytes.TrimSpace(raw)
+	if len(trimmed) == 0 || bytes.Equal(trimmed, []byte("null")) {
+		return config{}, nil
+	}
+	var cfg config
+	if err := json.Unmarshal(trimmed, &cfg); err != nil {
+		return config{}, err
+	}
+	return cfg, nil
+}
+
+func init() {
+	builtin.Register(Factory{})
+}