fix: async correctness to avoid blocking event loop and user cache saving update

[leonace924]

Validator async: avoid blocking the event loop

The validator scoring pipeline is async (e.g. gittensor/validator/evaluation/reward.py:get_rewards()), but several underlying GitHub API helpers use synchronous requests.
This can block the asyncio event loop, preventing other coroutines (and future timeout/cancellation logic) from making progress.

What was changed

• We implemented the fastest retrofit: run synchronous GitHub HTTP calls in a background thread via asyncio.to_thread().
• Added validation check before caching GitHub user data to ensure only complete user objects with essential fields are stored in the cache.

Updated call sites

• gittensor/validator/evaluation/reward.py:reward()
  • validate_response_and_initialize_miner_evaluation() is executed via asyncio.to_thread() (it calls GitHub /user under the hood).
  • get_user_merged_prs_graphql() (GraphQL POSTs) is executed via asyncio.to_thread().
  • PR file diff retrieval is now done through an async scoring function (below).
• gittensor/validator/evaluation/scoring.py
  • Added score_pull_requests_async(), which calls get_pull_request_file_changes() (REST GETs) via asyncio.to_thread().
  • Kept the existing synchronous score_pull_requests() for compatibility with scripts/tests.

Safety fix included

• gittensor/validator/evaluation/inspections.py:validate_response_and_initialize_miner_evaluation() no longer dereferences response.axon.hotkey before checking whether response is present.
  This prevents crashes when a miner does not respond.

Validation check for user data

In gittensor/utils/github_api_tools.py (lines 155-156), the code now validates that user data contains both id and created_at fields before caching:

Why to_thread() and not aiohttp

aiohttp is the cleaner long-term approach, but switching requires rewriting the GitHub API layer and retry/rate-limit logic.
asyncio.to_thread() is low-risk and immediately prevents the event loop from being blocked by requests calls.

[leonace924]

@anderdc would you review my PR, please?