fix(slack-gateway): log oauth callback failures

[onutc]

Summary

• log Slack OAuth callback failures at the exact failing stage
• capture backend/upsert error details in the existing gateway logger without logging Slack codes or bot tokens
• cover the new logging behavior with a regression test

Testing

• cd /Users/onur/repos/spritz/integrations/slack-gateway && go test ./... -run TestOAuthCallbackReturnsBadGatewayWhenBackendUpsertFails -count=1
• cd /Users/onur/repos/spritz/integrations/slack-gateway && go test ./...

[gitrank-connector]

👍 GitRank PR Analysis

Score: 5 points

Metric	Value
Component	Other (1× multiplier)
Severity	P3 - Low (5 base pts)
Final Score	5 × 1 = 5

Eligibility Checks

Check	Status
Issue/Bug Fix	✅
Fix Implementation	✅
PR Documented	✅
Tests	✅
Lines Within Limit	✅

Impact Summary

This PR adds comprehensive error logging to the Slack OAuth callback handler at multiple failure points (state validation, code exchange, installation upsert) and includes a success log. The changes improve observability and debuggability of OAuth flow failures while safely excluding sensitive tokens. A regression test was added to verify the logging behavior when backend upsert fails.

Analysis Details

Component Classification: This PR affects the Slack OAuth gateway integration, which doesn't map to a specific component in the provided table. It falls under OTHER as a miscellaneous integration improvement.

Severity Justification: This is a P3 (Low) severity fix. It adds observability/logging to OAuth callback failures without fixing a critical bug or preventing data loss. The logging helps with debugging but doesn't resolve a broken feature or security vulnerability.

Eligibility Notes: issue=true: PR title indicates 'fix' and addresses logging of failures. fix_implementation=true: Code changes align with PR description, adding logging at each failure stage. pr_linked=true: Clear summary and testing instructions provided. tests=true: Test file modified with new assertions. tests_required=true: Bug fixes in business logic (even observability fixes) require tests to prevent regression.

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