Tool call result metadata lost when retrieving session via server API

[trungutt]

Problem

When a tool call completes during streaming, the tool_call_response event includes rich result information:

// pkg/runtime/event.go
type ToolCallResponseEvent struct {
    Type           string                `json:"type"`
    ToolCall       tools.ToolCall        `json:"tool_call"`
    ToolDefinition tools.Tool            `json:"tool_definition"`
    Response       string                `json:"response"`
    Result         *tools.ToolCallResult `json:"result,omitempty"`  // ← Contains isError, meta
    AgentContext
}

// pkg/tools/tools.go
type ToolCallResult struct {
    Output  string `json:"output"`
    IsError bool   `json:"isError,omitempty"`  // ← Lost in REST API
    Meta    any    `json:"meta,omitempty"`     // ← Lost in REST API
}

However, when retrieving a session via GET /sessions/:id, the response only contains chat.Message which stores tool responses as:

// pkg/chat/chat.go - Message struct
Content    string `json:"content"`      // Only the output string
ToolCallID string `json:"tool_call_id"` // Reference to tool call
// No isError field
// No meta field

The example above is for result in tool_call_response. But there are also other attributes as well, such as toolDefinition.

Ideally we should be able to have the same level of information between events vs API