fix: replace deprecated asyncio.get_event_loop() and deduplicate network init

[t7929375-eng]

Problem

Both miner_post and miner_check CLI commands use asyncio.get_event_loop().run_until_complete() which is deprecated since Python 3.10 (emits DeprecationWarning) and raises RuntimeError in Python 3.12+ when no running event loop exists.

Additionally, both commands duplicate the bittensor initialization block (wallet, subtensor, metagraph, dendrite) — once inside console.status() and once outside for JSON mode — making the code fragile and harder to maintain.

Solution

1. Replace asyncio.get_event_loop().run_until_complete(coro) with asyncio.run(async_fn()) — the modern equivalent that properly creates and manages the event loop.
2. Extract _connect() helper to eliminate the duplicated 4-line init block in each command.
3. Extract _broadcast() / _check() async helpers to avoid repeating the dendrite call.

Changes

post.py

• Extract _connect() for wallet/subtensor/metagraph/dendrite init (−8 duplicated lines)
• Extract _broadcast() coroutine, replace 2× asyncio.get_event_loop().run_until_complete()

check.py

• Same _connect() extraction (−8 duplicated lines)
• Extract _check() coroutine, replace 2× asyncio.get_event_loop().run_until_complete()

Net result: +38 / −48 lines (code reduction).

Type of Change

• Bug fix
• Refactor

Testing

• ruff check passes
• ruff format passes
• pyright passes (0 errors)
• Self-reviewed

Checklist

• Code follows project style guidelines
• Self-review completed

cc @anderdc @LandynDev