fix(webhook): fail missing PR file content blobs

[JSONbored]

Summary

• Add typed GraphQL blob handling for PR file-content fetches.
• Fail the current content batch when a required base/head blob alias is missing from a successful GraphQL response.
• Keep expected nullable content behavior for added files, removed files, binary blobs, and oversized blobs.

Why

GitHub GraphQL can return null for object(expression: ...) without top-level errors. A live probe against this repo confirms that a missing object expression returns:

{"data":{"repository":{"missingObject":null}}}

Before this change, a modified text file could be stored with a missing required head or base blob as trusted null content. That lets the file fetch complete even though a required file-content input was absent.

This keeps the existing retry/failure path responsible for incomplete PR file content instead of persisting an incomplete row as if it were valid.

Prior context

• [Bug] PR file fetch can mark scoring data complete after content fetch is skipped #37 / fix: fail incomplete pr content fetches #38 already made incomplete content fetches fail for missing headSha and exhausted GraphQL batch failures. This covers the remaining successful-response case where an individual required GraphQL alias resolves to null.
• [Bug] Deleted PR files are marked complete without base content #20 / Fetch base content for deleted PR files #21 established that missing content is valid only for the optional side of added/removed files. This preserves those semantics while rejecting missing required sides.
• fix(webhook): fail GraphQL responses with errors #78 made GraphQL response-level errors fail. This handles the no-errors, null-alias shape separately.
• [Bug] Failed PR_FILES jobs can block future file refreshes #124 / fix(queue): evict failed PR_FILES jobs #128 made failed PR_FILES jobs retryable by not retaining failed jobs. This change intentionally uses that failure path when required content is absent.

No existing issue or PR was found for this exact per-alias GraphQL null-object case.

Type of Change

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

Testing

• npm run format:check
• npm run lint
• npm run build
• git diff --check
• Local missing-blob harness verified a required head blob returning null throws before any content upsert
• Live GraphQL probe verified missing object(expression: ...) returns null without top-level errors
• Codex Security diff scan: 0 reportable findings
• CodeRabbit CLI review: 0 issues

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable)
• Documentation updated (if applicable)
• No unnecessary dependencies added