A successful tool result without data strands the turn (TypeError in spillOrTruncate) #229

Found while reviewing #220. Present on main (e457abd).

What happens. ToolResult.data is optional, but a successful result without it throws while the result is being stored, before the tool_result is handed back to the stream:

• toolResultDataToHistoryString(undefined) returns JSON.stringify(undefined), i.e. undefined, despite its string return type:

  agent-framework/src/tool-result-history.ts

  Line 55 in e457abd

  const str = fromArray ?? JSON.stringify(data);

• spillOrTruncate then reads content.length:

  agent-framework/src/framework.ts

  Line 11447 in e457abd

  if (!cap || content.length <= cap) return { text: content, filePath: null };

TypeError: Cannot read properties of undefined (reading 'length')
    at AgentFramework.spillOrTruncate
    at AgentFramework.buildStoredToolResultContent
    at AgentFramework.handleProcessEvent

Repro. A module tool that returns { success: true }; one mock-membrane turn calling it. The agent stays in ready, the stream never receives tool results, the tool_use is already persisted, and framework.stop() then hung in my probe. Same on main and on #220's head.

The MCPL path can produce the same shape: a successful tools/call with content: [] (or only empty text blocks) maps to data: undefined:

Direction. Normalize absent data to a stable string (e.g. '' or '(no output)') in toolResultDataToHistoryString, and make the result-storage step fail the one call rather than the event, so a bad result still pairs its tool_use.