[codex] Fix AI gateway dashboard loading errors

[Pigbibi]

Summary

• Fix AiGateway service GET routing so query-string read endpoints like /v1/ai/changes/effectiveness?days=90 reach the intended handler.
• Prevent one dashboard API failure from leaving all cards in perpetual loading by rendering per-panel error states.
• Center the loading spinner visually in dashboard cards.

Root cause

AiGatewayRequestHandler.do_GET() matched routes against self.path, which includes the query string. Requests with ?days=90 missed the exact /v1/ai/changes/effectiveness route and fell through to an invalid change lookup. The dashboard then used Promise.all, so any failed API request prevented all panels from rendering.

Validation

• python3 -m ruff check .
• python3 -m pytest tests -q
• npx -y node@22 --experimental-default-type=module --test cloudflare/codex-audit-proxy/tests/index.test.mjs
• npx -y node@22 --experimental-default-type=module --test cloudflare/ai-gateway-dash/tests/index.test.mjs
• git diff --check

[github-actions]

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