test(ai): lock JSON mode forwarding for ToolLoopAgent object output

[giulio-leone]

Summary

• add regression tests proving ToolLoopAgent.generate() forwards Output.object() as responseFormat: json
• add regression tests proving ToolLoopAgent.stream() forwards Output.object() as responseFormat: json
• this complements existing coverage already present for generateText, streamText, and streamObject

Context

Issue #12491 reports missing provider-level JSON mode with generateText + Output.object(). This PR hardens the agent layer to ensure no regression in the agent.generate() / agent.stream() path.

Validation

• pnpm --filter ai exec vitest --config vitest.node.config.js --run src/agent/tool-loop-agent.test.ts
• pnpm --filter ai exec eslint src/agent/tool-loop-agent.test.ts
• pnpm --filter ai type-check
• repeated the same validation suite twice consecutively with zero failures

Closes #12491

Attribution request

If this pull request is merged, I would be grateful if contributor credit could be included in the related changelog or release notes for implementing this fix (@g97iulio1609).