LocalReplyConfig Body Rewrite Generates Null Bytes in Cilium Envoy

When using Cilium Envoy's LocalReplyConfig for response body rewriting, configuring body with inline_string content results in the response body containing null bytes (0x00) equal in length to the specified string, rather than the expected text content.

Cilium Version：1.18.6
Kubernetes Version: 1.29.9

local_reply_config:
  mappers:
  - filter:
      status_code_filter:
        comparison:
          op: EQ
          value:
            default_value: 404
    body_format_override:
      text_format: "Custom 404: %REQ(:path)% not found"
  body_format:
    text_format: "Default error: %RESPONSE_CODE%"

Expected Behavior
When the configured status code (e.g., 404) is triggered, the response body should contain the configured custom text message.

Actual Behavior
The response body contains null bytes (0x00) equal in length to the configured string. For example:

Configured string: "Custom 404: /api not found" (24 characters)

Actual response: 24 null bytes

Start by reproducing the LocalReplyConfig response-body rewrite with the supplied Cilium and Kubernetes versions, using the shown mappers and inline text configuration. Trace how the configured body text is converted before Envoy sends the response. Done means a triggered 404 returns the configured text, including the path, rather than same-length null bytes.