feat(realtime): add unified set() method for prompt + image

[AdirAmsalem]

Summary

• Adds a unified set() method to RealtimeClient that atomically sends prompt and/or image in a single wire message (type: "set" / set_ack)
• Uses a sentinel pattern (_Unset) to distinguish "not provided" (omit from wire) from None (clear the value)
• Uses Pydantic's exclude_unset=True serialization so only explicitly set fields are sent on the wire
• Keeps existing set_prompt() and set_image() for backward compatibility

API

# Prompt only
await client.set(prompt="a cat in space")

# Prompt + enhance
await client.set(prompt="a cat", enhance=True)

# Image only (bytes, Path, URL, or file-like)
await client.set(image=Path("ref.png"))

# Prompt + image together (atomic)
await client.set(prompt="a cat", image=b"...")

# Clear image
await client.set(image=None)

Wire Protocol

→ {"type": "set", "prompt": "a cat", "enhance_prompt": true, "image_data": "base64..."}
← {"type": "set_ack", "success": true, "error": null}

Omitted fields = no change. null = clear.

Changes

• decart/realtime/messages.py — Added SetParamsMessage, SetAckMessage, updated message unions, exclude_unset serialization
• decart/realtime/webrtc_connection.py — Added set_ack handling, register_set_wait/unregister_set_wait
• decart/realtime/webrtc_manager.py — Pass-through for set wait registration
• decart/realtime/client.py — Added set() method with sentinel pattern, validation, image conversion
• tests/test_realtime_unit.py — 11 new tests (93 total passing)
• examples/ — Updated to use set() as primary API

Companion to JS SDK PR: DecartAI/sdk#76