fix: return JSON response for unmatched routes instead of plain text

[cemalkilic]

What kind of change does this PR introduce?

Fix

Summary

Routes behind feature flags (e.g. OAuthServer, CustomOAuth) return chi's default plain text "404 page not found" when disabled. Register custom NotFound and MethodNotAllowed handlers on the router so all unmatched routes return structured JSON errors consistent with the rest of the API.

Fixes inconsistent content-type: text/plain responses when feature-gated routes (OAuthServer, CustomOAuth) are disabled

Test plan

• Hit a disabled route (e.g. /oauth/clients/register with OAuthServer.Enabled=false) and verify JSON 404 response with error_code: "route_not_found"

[fadymak]

LGTM ✅ Might be worth codifying this with a small test