Reject missing repository mappings

[skyfallwastaken]

Summary of the problem

GitHub returns 404 for both nonexistent repositories and private repositories hidden from a limited OAuth token. The repository mapping verifier treated limited-scope 404 responses as ambiguous and failed open, so typos such as https://github.com/hackcl/hackatime were accepted :(

Describe your changes

Treat every GitHub repository API 404 as nonexistent or inaccessible

Screenshots / Media

N/A

[Copilot]

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[greptile-apps]

Greptile Summary

This PR makes repository mapping validation reject every GitHub API 404 while continuing to fail open for other API and network failures.

• Removes OAuth-scope-dependent handling of GitHub 404 responses.
• Adds regression coverage for rejecting the reported hackcl/hackatime typo.
• Removes coverage preserving mappings for private repositories hidden from limited tokens.

Confidence Score: 5/5

The PR appears safe to merge because the stricter handling matches the stated accessibility requirement and includes focused regression coverage.

GitHub 404 responses now deterministically block mappings, while transient, authentication, rate-limit, and server failures retain their existing fail-open behavior.

Important Files Changed

Filename	Overview
app/services/repo_host/github_service.rb	Treats every repository lookup returning 404 as nonexistent or inaccessible and removes the obsolete OAuth-scope helper.
test/models/project_repo_mapping_test.rb	Updates model coverage to verify that the reported nonexistent GitHub repository is rejected.

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