✨ Implement full support for nested fields in Meta and Entry marshalling

This commit completes the implementation of nested field support.
- :
    -  now correctly handles  and  by recursively flattening them into  format.
    - Introduced  for recursive map marshalling.
    - Refactored  for cleaner buffer writing.
- : Added comprehensive test cases for nested JSON, nested maps, double-nested maps, and nested keys within JSON to ensure correct marshalling and unmarshalling.
- : Updated tests to reflect the new nil handling and removed redundant JSON object test.

This allows for more flexible and structured data representation within log entries.

models/meta_test.go

@@ -35,12 +35,11 @@ func TestMeta(t *testing.T) {
 		{"json number", "num", json.Number("42.13"), "@num 42.13", nil, 42.13},
 		{"true", "b", true, "@b true", nil, true},
 		{"false", "b", false, "@b false", nil, false},
-		{"nil", "n", nil, "", nil, ErrorParsing},
+		{"nil", "n", nil, "", ErrorParsing, ErrorParsing},
 		{"time", "when", when, "@when " + when.Format(time.RFC3339), nil, when},
 		{"rune", "char", '@', "@char @", nil, "@"},
 		{"bytes", "byteme", []byte("yo"), "@byteme yo", nil, "yo"},
 		{"byte", "byteme", byte(67), "@byteme C", nil, "C"},
-		{"json-obj", "obj", json.RawMessage(`{"foo":"bar","baz":"quux"}`), `@obj {"foo":"bar","baz":"quux"}`, nil, json.RawMessage(`{"foo":"bar","baz":"quux"}`)},
 		{"json-arr", "arr", json.RawMessage(`["foo",42,"bar", null,"quux", true]`), `@arr ["foo",42,"bar", null,"quux", true]`, nil, json.RawMessage(`["foo",42,"bar", null,"quux", true]`)},
 		{"chan", "nope", make(chan bool), "", errors.New("Unsupported type chan bool"), ""},
 		{"whitespace-key", "no space", "hi", "", errors.New("whitespace is not allowed in key: no space"), ""},