fix(security): bound fan-out and method surface and scrub relayed text

- Add a process-wide gate on MCP tools/list backend connections. The
  per-request limit alone let N callers open N times as many.
- Reject CONNECT and TRACE on native passthrough and on MCP and A2A
  targets with 405.
- Scrub URL query strings and the target URL from a backend JSON-RPC
  error message before it reaches the caller.
- Replace the value of redis.address in GET /admin/api/config. The
  users file path stays visible by design.
- Escape line breaks in every log line so an upstream string cannot
  forge log entries.
- Add stripClientAnthropicBeta. When true, POST /v1/messages does not
  forward the client anthropic-beta header. The default stays false.

admin_config_test.go

@@ -107,7 +107,7 @@ func TestRedactConfig_LiteralAndIndirectSecrets(t *testing.T) {
 	}
 	body := string(raw)
 
-	for _, leaked := range []string{"literal-key", "redis-literal-pw", "alice-literal-key", "s3cr3t", "user:pw", "abc123"} {
+	for _, leaked := range []string{"literal-key", "redis-literal-pw", "alice-literal-key", "s3cr3t", "user:pw", "abc123", "redis:6379"} {
 		if strings.Contains(body, leaked) {
 			t.Errorf("redacted config still contains %q: %s", leaked, body)
 		}