fix(lsp): correct Content-Length in BDD scenario from 46 to 43 bytes

Body is exactly 43 bytes long. CL=46 caused _read_one_message to timeout
waiting for 3 extra bytes, returning None instead of valid JSON.

features/lsp_header_injection_security.feature

@@ -26,7 +26,7 @@ Feature: LSP transport header injection vulnerability (issue #7112)
     And the error message must contain "non-ASCII"
 
 @tdd_issue_7112 Scenario: Valid ASCII headers are processed correctly
-    Given a Transport mock with BytesIO stream containing b"Content-Length: 46\r\n\r\n{\"jsonrpc\":\"2.0\",\"id\":1,\"result\":\"success\"}"
+    Given a Transport mock with BytesIO stream containing b"Content-Length: 43\r\n\r\n{\"jsonrpc\":\"2.0\",\"id\":1,\"result\":\"success\"}"
     When _read_one_message() is invoked
     Then it should return a parsed JSON dict
     And the result must contain "jsonrpc" == "2.0"