Fix webhook authorization when AUTH_ENABLED=true

[kamphaus]

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• I have read through the Contributing Documentation.
• I have added relevant tests.
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Summary

When AUTH_ENABLED=true, requests to REST-style webhook endpoints (e.g. /rest/plugins/webhook/connections/:id/deployments) were incorrectly rejected with 401 Unauthorized even when a valid, authorized API key was supplied. The root cause is in RestAuthentication: it validates the API key and stores the resulting user on the gin context, then reroutes the request from /rest/... to /plugins/... via gin's HandleContext. That call internally resets c.Keys, wiping the user that was just set, so the downstream RequireAuth middleware sees no authenticated user and rejects the request.

The fix stores the authenticated user in the underlying http.Request's context as well, since c.Request survives gin's context reset during the reroute. shared.GetUser now falls back to this request-context value when the gin context key is missing. A regression test spins up a real router with RestAuthentication + a RequireAuth-style gate + a webhook handler, and confirms a valid bearer-token request now reaches the handler (200) instead of being rejected (401).

Does this close any open issues?

Closes #9007

Other Information

• Disclose AI usage: AI-assisted
• I have verified that the regression test fails without the fix and passes with the fix.