chore(deps): upgrade fetch-mock to v12

[LuisUrrutia]

Summary

• Upgrade fetch-mock from @gr2m/[email protected] fork to [email protected]
• Migrate all test files to fetch-mock v12 API

Changes

• sandbox() → createInstance()
• *Once() methods → *() with { repeat: 1 } option
• fetch variable → fm.fetchHandler when passing to Octokit
• fetch.done() → fm.callHistory.done()
• Function matcher signature (url, options) → ({ url, options })
• Function matchers now return false to reject requests instead of using assertions, since v12 requires explicit rejection for route fallthrough

Test plan

• All tests pass