fix: escape special characters in GraphQL file path expressions

[t7929375-eng]

Summary

File paths containing double quotes or backslashes are interpolated directly into GraphQL expression fields (e.g. {sha}:{path}), producing syntactically invalid queries. This causes _fetch_file_contents_batch and _fetch_file_contents_with_base_batch to silently return empty results for affected files, giving those PRs a score of 0.

Adds a _escape_graphql_string helper and applies it to all three path interpolation sites:

• _fetch_file_contents_batch (head expression)
• _fetch_file_contents_with_base_batch (base expression)
• _fetch_file_contents_with_base_batch (head expression)

Related Issues

Fixes #368

Type of Change

• Bug fix

Testing

• ruff check passes
• ruff format passes
• pyright passes (0 errors)
• Self-reviewed

Checklist

• Code follows project style guidelines
• Self-review completed

cc @anderdc @LandynDev