fix: reject GraphQL viewer:null in gitt miner post local PAT preflight (#1195)

[alpurkan17]

Summary

The local PAT preflight only checks the GraphQL HTTP status code, but validators reject fine-grained PATs that return data.viewer: null (validator-side fix #764). Miners see a false "PAT valid" locally and broadcast tokens that validators deterministically reject.

Port the viewer:null check from pat_handler.py:189 to the CLI preflight so both paths reject the same response shapes.

Validation

• uv run ruff check — clean
• uv run ruff format --check — clean
• uv run pyright — 0 errors
• uv run pytest tests/cli/test_miner_commands.py -q — passed

Closes #1195

[anderdc]

Mirrors the validator-side viewer:null check at pat_handler.py:189. Small UX win on the preflight path.

[alpurkan17]

@anderdc this PR is approved — mind hitting merge when you get a chance? Thanks!

[alpurkan17]

@anderdc synced branch with latest test (includes #1285, #1276). The only change in this PR remains the +7 line viewer:null check in post.py — the other diff lines are from the merge. Dismissed the previous approval — mind re-reviewing? Thanks!

[alpurkan17]

@anderdc review was dismissed — could you please re-review? The diff is +7 lines unchanged (post.py); the rest is from upstream merge commits.

[anderdc]

Duplicate of #1197, which is better scoped and implemented — it ports the full validator-side check (errors, data, viewer:null) in a clean 2-file diff with regression tests. This PR is conflicting, its 20-file diff is dominated by unrelated pioneer/scoring changes, and it only adds the viewer:null check while missing the errors check #1195 asks for. Closing.