Fix JsonEncoder not using serde for Serialize types

[axd1x8a]

When using the JsonEncoder, values captured for serde serialization (e.g., key:serde = &value) are incorrectly logged as their Debug or Display string representation, rather than as a nested JSON object.

Example:

A log statement like this:

log::info!(
    response:serde = &MyStruct { id: 1 };
    "Request completed"
);

Produces this output:

{
  "attributes": {
    "response": "MyStruct { id: 1 }"
  },
  ...
}

Instead of the expected output:

{
  "attributes": {
    "response": { "id": 1 }
  },
  ...
}

[estk]

@feeeek can you please update the branch?

[axd1x8a]

@estk done