[6.2] Request multi-tenancy configuration as JSON instead of parsing HOCON

[Kinplemelon]

Summary

The Dashboard currently loads the multi-tenancy configuration through GET /configs?key=multi_tenancy and parses the HOCON response on the frontend.

When post_auth_tns_expression contains quotation marks, EMQX can represent the value using its multiline HOCON syntax ("""~...~"""). Handling that custom syntax in the Dashboard can cause the saved value to be displayed incorrectly when the configuration drawer is reopened.

Request the configuration in JSON format instead by sending Accept: application/json, then consume the JSON response directly.

Expected behavior

• The multi-tenancy configuration request asks EMQX for JSON.
• A saved post_auth_tns_expression containing quotation marks is returned and displayed without modification.
• Reopening the namespace configuration drawer preserves values such as "abcd" and coalesce(client_attrs.tag, "default").

Additional context

• Backend report and API guidance: [6.2.2]There is a formatting display issue when entering a value containing double quotes for post_auth_tns_expression. like "abcd" emqx#17836
• Original Dashboard display bug (closed): [6.2.2]When the post_auth_tns_expression configuration contains quotation marks, there is a display error on the Dashboard after saving. #3754

[zmstone]

maybe older version needs fix if HOCON config string is parsed.