test: simplify GitHub CLI mock responses

[setchy]

Summary

Replace the nested response-selection ternary in the GitHub CLI authentication test mock with straightforward if/else if branches.

Preserve rejected-credential responses, user identity responses, empty successful responses, status codes, headers, and request recording.

Addresses the sonarjs/no-nested-conditional finding from the one-off test-code scan. Only src/renderer/utils/forges/github/cli-auth.test.ts changes; no production authentication behavior, dependencies, or configuration changes.

Validation

• Focused GitHub CLI credential-lifecycle suite: 6 tests passed.
• Full suite: 1,865 tests passed across 186 files.
• TypeScript, formatting/lint checks, and production build passed. The build emitted a non-fatal chunk-size warning.
• Type-aware SonarJS recommended-rule scan of the changed file: zero findings and zero parsing errors. This is a local ESLint-plugin result, not a claim about SonarCloud issue closure.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud