fix: use async ensure_opg_approval in llm_chat example

[verseon0980]

The example file llm_chat.py demonstrates usage of ensure_opg_approval() inside an async function without using await.

async def main():
llm = og.LLM(private_key=...)
llm.ensure_opg_approval(min_allowance=0.1) # synchronous call
result = await llm.chat(...)

This is problematic because:

ensure_opg_approval() is a synchronous, blocking function
It internally performs polling using time.sleep()
When called inside async def, it blocks the entire asyncio event loop

Impact

Any developer copying this official example will unknowingly introduce:

Frozen event loop execution
Blocked async tasks and background operations
Delayed or broken network I/O
Inability to handle timeouts or cancellations properly

This leads to incorrect async usage patterns being propagated via official documentation.

Root Cause

The example uses a synchronous function inside an async context:

llm.ensure_opg_approval(min_allowance=0.1)

Since this function blocks (via time.sleep() internally), it prevents the event loop from scheduling other coroutines.

Fix

Update the example to use the newly introduced async-safe method:

async def main():
llm = og.LLM(private_key=os.environ.get("OG_PRIVATE_KEY"))
await llm.ensure_opg_approval_async(min_allowance=0.1)
result = await llm.chat(...)

Why this works
ensure_opg_approval_async() uses asyncio.sleep()
This allows the event loop to continue running other tasks
Ensures the example is correctly aligned with asyncio best practices