fix(auth): harden callback relay registration retries

[xjdr-noumena]

Summary

Supersedes #55 with the same callback-relay reliability fix plus explicit retry-boundary coverage.

The callback-relay registration POST used a 1000ms timeout, which can be shorter than cold DNS/TLS on a fresh process. This moves retry behavior into registerOauthCallbackRelay, raises the service-level registration budget to 5000ms, and retries only transient registration failures.

Behavior

• retries timeout/network-style Axios errors
• retries 5xx and 429 responses
• fails fast on non-transient 4xx responses
• fails fast on non-Axios/programming errors
• keeps the OAuth service layer simpler by making callback-relay registration resilient at the client boundary

Validation

Focused OAuth tests:

PATH="$HOME/.bun/bin:$PATH" timeout 60s $HOME/.bun/bin/bun test \
  src/services/oauth/client.test.ts \
  src/services/oauth/oauthServiceE2E.test.ts

Result: 17 pass, 0 fail.

Auth suite:

PATH="$HOME/.bun/bin:$PATH" timeout 120s $HOME/.bun/bin/bun run test:auth

Result: 457 pass, 0 fail.

Notes

This keeps the spirit of #55 but tightens the retry classifier. Non-Axios errors are not retried, because those are more likely programming or invariant failures than transient network failures.