Sanitize go-github ErrorResponse to prevent panic in SDK error handling

[itsmylife]

Summary

Fixes a nil pointer dereference panic caused by the interaction between go-github v81's
ErrorResponse.Is and the SDK's guessErrorStatus function.

go-github v76 (bumped from v72 to v81 in #574) changed ErrorResponse.Is from a simple type assertion
to calling errors.As(target, &v) on the target. See PR: google/go-github#3739
The SDK's guessErrorStatus
(backend/status.go:112) passes typed nil targets ((*url.Error)(nil)) to errors.Is.
When errors.Is walks the error chain and reaches *github.ErrorResponse, the v81
ErrorResponse.Is calls errors.As on that typed nil, which tries to call Unwrap() on
the nil receiver — causing a nil pointer dereference panic.

The fix adds sanitizeGitHubError which converts *github.ErrorResponse to a plain error
at the addErrorSourceToError boundary, preserving the error message while stripping the
problematic Is method from the error chain.

Manual reproduction steps

1. Run Grafana with the github-datasource plugin (v2.5.0)
2. Configure the GitHub datasource with a valid personal access token
3. Open Explore (or create a panel) and select the GitHub datasource
4. Set query type to Workflows (or Code Scanning)
5. Set Owner to grafana, Repository to this-repo-does-not-exist
6. Run the query
7. Observe the panic in Grafana server logs:
   panic triggered error="runtime error: invalid memory address or nil pointer dereference"

With this fix applied, the query returns a proper error response instead of panicking.

Fixes https://github.com/grafana/oss-big-tent-squad/issues/183

[zoltanbedi]

LGTM