fix: prevent AI JSON parse from leaving request stuck in isWorking

[nothariharan]

Summary

• Fixes non-streaming AI responses getting stuck with isWorking: true when response JSON decode fails (#1741).
• Only decode non-empty bodies, wrap kJsonDecoder.convert / formatting in try/catch, and keep the raw response on failure so isWorking: false still runs.

Test plan

• Create a non-streaming AI request that returns HTTP 200 with invalid JSON (or empty body).
• Send the request and confirm the UI leaves the working/cancel state (Send is usable again).
• Confirm a valid JSON AI response still gets formattedBody as before.
• Confirm a normal HTTP (non-AI) request path is unchanged.

Notes

• Small, focused bugfix only — no dependency bumps or refactors.
• AI-assisted for investigation/drafting; change reviewed and understood before submit.
• No new automated test yet (provider sendRequest path is heavy to scaffold); happy to add one if maintainers want it.

Fixes #1741