[Bug] Three gaps in #769 stale-closed PR fix: scoring data loss, silent drop path, and fragile test

[dataCenter430]

Description

PR #769 introduced the stale_closed_pull_requests bucket to refresh pr_state for out-of-window PRs but left two issues unaddressed.

Gap 1 — Stale UPSERT silently zeroes previously-scored rows
store_pull_requests_bulk in gittensor/validator/utils/storage.py:74-76 uses BULK_UPSERT_PULL_REQUESTS, an ON CONFLICT DO UPDATE SET that overwrites all columns via EXCLUDED.*. PullRequest.from_graphql_response() leaves every scoring field at its dataclass default (base_score = 0.0, earned_score = 0.0, credibility_multiplier = 1.0, etc.), so any row that was previously scored has those computed values silently replaced with zeros on every subsequent scan.

Gap 2 — Fragile positional assertion in test_stale_closed_prs_are_stored_separately
tests/validator/utils/test_storage_mirror.py:176 asserts stale storage via a hardcoded call index call_args_list[3]. Any reordering of bulk calls in store_evaluation silently breaks the assertion's target without failing the test.

Steps to Reproduce

1. A miner has a PR that was previously scored (e.g. was OPEN with collateral) and later closed outside the 35-day lookback window.
2. On the next validator scan, try_add_open_or_closed_pr routes it to stale_closed_pull_requests.
3. store_pull_requests_bulk fires the full BULK_UPSERT_PULL_REQUESTS with dataclass-default scoring fields.
4. The previously-computed earned_score, base_score, credibility_multiplier, etc. are overwritten with 0.0 / 1.0 in the database.

Expected Behavior

• Only pr_state and updated_at should be modified for stale PRs. All scoring columns on existing rows must be left untouched.
• The stale storage test should assert against the dedicated stale method by name, not by positional call index.

Actual Behavior

• Previously-scored rows have their scoring columns zeroed out by the stale UPSERT on every subsequent scan.
• test_stale_closed_prs_are_stored_separately asserts via call_args_list[3] and would pass incorrectly if call order in store_evaluation changed.

Environment

• OS: — Linux
• Python version: — 3.13
• Commit/Version: 86390ad (post-fix: sync pr_state for stale-closed PRs excluded from scoring #769 merge)

Additional Context

Follow-up to #769, #768. No schema changes are required to fix either gap. Gap 1 can be resolved with a dedicated method backed by a targeted UPDATE touching only pr_state and updated_at instead of routing through the full-column UPSERT path.