fix: define constant for duplicated GQL client error string (go:S1192)

[devin-ai-integration]

Closes: SonarQube issue AZhX1hJ3UiHWiWeaJF-T (rule go:S1192)

Summary

Defines a package-level constant errFailedToGetGQLClient for the string literal "failed to get GitHub GQL client: %v" which was duplicated 4 times across the discussion handler functions in pkg/github/discussions.go.

This is a pure refactor with no behavioral change — the formatted error messages remain identical at runtime.

Changes

• Added unexported constant errFailedToGetGQLClient at the top of discussions.go
• Replaced all 4 inline occurrences in ListDiscussions, GetDiscussion, GetDiscussionComments, and ListDiscussionCategories

Human Review Checklist

• Confirm the constant value exactly matches the original literal (no typo or subtle difference)
• Confirm all 4 call sites pass err as the format arg — no change in error formatting behavior

Link to Devin session: https://app.devin.ai/sessions/a45f9eeb845a4e87b5d03ed42e1b90f7
Requested by: @parkerduff

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[devin-ai-integration]

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[sonarqubecloud]

Quality Gate failed

Failed conditions
D Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[devin-ai-integration]

The SonarCloud Quality Gate failure is due to 7 pre-existing godre:S8205 issues (nested anonymous structs) that were already in discussions.go before this PR. SonarCloud attributes them to this PR because the file was modified.

These are MINOR-severity issues unrelated to the go:S1192 fix in this PR. The SonarCloud check is also not marked as required, so the PR remains mergeable.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.