fix(github): guard backfill GraphQL helpers' repoFullName parsing

[philluiz2323]

Summary

fetchLiveCiAggregateViaGraphQl, fetchLivePullRequestReviewDecision, and fetchLiveReviewThreadBlockers in src/github/backfill.ts parsed repoFullName with a bare split(/) + truthiness check. Extra path segments (owner/repo/extra) and whitespace-padded slugs (owner/ repo, owner/repo) therefore reached GraphQL query strings — the same failure mode #8311 closed for app.ts/comments.ts.

Adds a local parseBackfillRepoFullName helper mirroring the segment-count + whitespace guard used in labels.ts/assignees.ts/pr-actions.ts, reused by all three call sites. Each function keeps its existing fail-soft return contract (null / undefined / []).

Test plan

• test/unit/graphql-status-rollup.test.ts: fetchLiveCiAggregateViaGraphQl rejects extra-segment and whitespace-padded slugs before any GraphQL call
• test/unit/backfill-2.test.ts: fetchLivePullRequestReviewDecision returns undefined for the same malformed shapes
• test/unit/backfill-2.test.ts: fetchLiveReviewThreadBlockers returns [] for the same malformed shapes
• Targeted vitest run passes (3/3 new cases)

Closes #9317

[codecov]

❌ 3 Tests Failed:

Tests completed	Failed	Passed	Skipped
11205	3	11202	16

View the top 2 failed test(s) by shortest run time

test/unit/selfhost-pg-retention.test.ts > runRetentionPrune + processJob on the Postgres backend (#977) > processJob prune-retention deletes eligible rows and records a success audit event on Postgres

Stack Traces | 0.00636s run time

AssertionError: expected 3 to be +0 // Object.is equality

- Expected
+ Received

- 0
+ 3

 ❯ test/unit/selfhost-pg-retention.test.ts:119:44

test/unit/selfhost-pg-retention.test.ts > pruneExpiredRecords on the Postgres backend (#977) > deletes across multiple bounded batches and stops at the per-table cap, same as the SQLite path

Stack Traces | 0.016s run time

AssertionError: expected +0 to be 4 // Object.is equality

- Expected
+ Received

- 4
+ 0

 ❯ test/unit/selfhost-pg-retention.test.ts:76:33

View the full list of 1 ❄️ flaky test(s)

test/unit/salvageability.test.ts > resolveAiReviewSalvageableHold > defaults: no configured floor uses the gate default, and a confidence-less blocker counts as certainty (at/above floor)

Flake rate in main: 100.00% (Passed 0 times, Failed 11 times)

Stack Traces | 0.00751s run time

AssertionError: expected undefined to be defined
 ❯ test/unit/salvageability.test.ts:88:18

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed · 🟪 Reviewing}

[philluiz2323]

Closing: opened while #9285 (green CI, fixes the salvageability + selfhost-pg-retention failures currently red on every open PR) is still merge-pending. Will re-file on a green main after that lands.