feat(realtime): add resetInput() method

[tomershlasky]

Summary

• Adds client.resetInput() to reset to passthrough mode at runtime (sends set_image(null, { prompt: null }) — same wire message as initial passthrough)
• Widens prompt type to accept null in set(), setImage(), and the manager chain
• No new wire protocol messages — reuses existing passthrough signal

Test plan

• Verify client.resetInput() switches back to passthrough after a prompt was set
• Verify client.set({ prompt: null }) works as equivalent
• Verify existing set(), setPrompt(), setImage() behavior unchanged

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/DecartAI/sdk/@decartai/sdk@96

commit: b8f713d