The dynamic OAuth GET handler swallowed every thrown error into a generic
`{ error: "Internal server error" }` 500, so a device-code upstream failure
(qwen → qwen.ai, codebuddy-cn → copilot.tencent.com — geo-block / outage / bad
client) surfaced in the extension as an indistinguishable 'ошибка сервера
OmniRoute', hiding WHY it failed.
Route the caught error through sanitizeErrorMessage() (already imported, hard
rule #12) so the real reason ("Device code request failed: …", "CodeBuddy
state request failed (403)") reaches the client, falling back to the generic
only when the sanitizer yields nothing.
Regression guard: tests/unit/oauth-device-code-error-transparency.test.ts
(source-level — the route needs the full Next request/auth/upstream graph, so
behavioural validation belongs on a real build/VPS).
