fix: include status code and response body in GitHub token error

[HarshMN2345]

No description provided.

[greptile-apps]

Greptile Summary

• Updates the GitHub App installation token error path in src/VCS/Adapter/Git/GitHub.php.
• Adds the HTTP status code to the token retrieval failure message.
• Limits response-body diagnostics to the allow-listed message and documentation_url fields.

Confidence Score: 5/5

The change is narrowly scoped to diagnostic text for GitHub installation token failures and does not alter the success path.

No code issues were identified in the modified error handling path, and the response-body exposure is constrained to explicit allow-listed fields.

T-Rex Logs

What T-Rex did

• Ran the test suite focused on the GitHub authentication path to verify access token retrieval behavior.
• Captured two log artifacts representing the base and head states of the GitHub access token retrieval flow.
• Compared the base and head outputs to note how the head commit adds HTTP status and a Bad credentials message to the exception.

_{Ran code and verified through T-Rex}

_{Reviews (2): Last reviewed commit: "fix: only expose safe fields from GitHub..." | Re-trigger Greptile}