Add JSON serialization to LLM message types

Message, Part (Text/Image/File), ToolCall, FunctionCall, and Usage
now round-trip through JSON. Message uses a type-discriminated
envelope for the Part interface. Required for checkpoint persistence.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/llm/message_test.go

@@ -0,0 +1,103 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package llm
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMessageJSONRoundTrip(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		msg  Message
+	}{
+		{
+			name: "text only",
+			msg: Message{
+				Role:  RoleUser,
+				Parts: []Part{TextPart{Text: "hello"}},
+			},
+		},
+		{
+			name: "image part",
+			msg: Message{
+				Role:  RoleUser,
+				Parts: []Part{ImagePart{URL: "https://example.com/img.png"}},
+			},
+		},
+		{
+			name: "file part",
+			msg: Message{
+				Role: RoleUser,
+				Parts: []Part{FilePart{
+					Data: "aGVsbG8=", MimeType: "text/plain", Filename: "hello.txt",
+				}},
+			},
+		},
+		{
+			name: "mixed parts",
+			msg: Message{
+				Role: RoleUser,
+				Parts: []Part{
+					TextPart{Text: "see attached"},
+					FilePart{Data: "aGVsbG8=", MimeType: "text/plain", Filename: "hello.txt"},
+				},
+			},
+		},
+		{
+			name: "assistant with tool calls",
+			msg: Message{
+				Role:  RoleAssistant,
+				Parts: []Part{TextPart{Text: "calling tool"}},
+				ToolCalls: []ToolCall{{
+					ID:       "call_1",
+					Function: FunctionCall{Name: "search", Arguments: `{"q":"test"}`},
+				}},
+			},
+		},
+		{
+			name: "tool response",
+			msg: Message{
+				Role:       RoleTool,
+				ToolCallID: "call_1",
+				Parts:      []Part{TextPart{Text: "result"}},
+			},
+		},
+		{
+			name: "empty parts",
+			msg:  Message{Role: RoleAssistant},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			data, err := json.Marshal(tt.msg)
+			require.NoError(t, err)
+
+			var got Message
+			err = json.Unmarshal(data, &got)
+			require.NoError(t, err)
+			assert.Equal(t, tt.msg, got)
+		})
+	}
+}