fix(cli): reject viewer-null PAT preflight responses

[volcano303]

Summary

• Reject GraphQL 200 PAT preflight responses when the body contains populated errors, missing/null data, or data.viewer: null.
• Keep the existing fine-grained PAT permission message for local GraphQL access failures.
• Add focused CLI regression coverage for valid viewer responses and body-level GraphQL rejections.

Related Issues

Closes #1195

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

Commands run:

uv run pytest tests/cli/test_miner_commands.py -q
uv run pytest tests/validator/test_pat_handler.py -q
uv run pytest tests/cli/test_miner_commands.py tests/validator/test_pat_handler.py -q
uv run ruff check gittensor/cli/miner_commands/post.py tests/cli/test_miner_commands.py
uv run ruff format --check gittensor/cli/miner_commands/post.py tests/cli/test_miner_commands.py
git diff --check

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)