[codex] Clean up core OAuth discovery errors

[RhysSullivan]

Summary

• remove unknown error message/stringification from OAuth helper and discovery failures
• inline OAuth discovery tagged errors instead of using a trivial factory
• keep native/provider details in typed cause fields while using stable public messages
• update OAuth helper/discovery tests to avoid native Error and tagged-error instanceof checks

Validation

• bun run typecheck in packages/core/sdk
• bunx vitest run src/oauth-helpers.test.ts src/oauth-discovery.test.ts in packages/core/sdk
• bunx oxlint -c .oxlintrc.jsonc packages/core/sdk/src/oauth-helpers.ts packages/core/sdk/src/oauth-discovery.ts packages/core/sdk/src/oauth-discovery.test.ts packages/core/sdk/src/oauth-helpers.test.ts --deny-warnings

Stacked on #535.

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commit: 0bbd756