fix(oauth): honor discovered CIMD support

[aryasaatvik]

Summary

Prefer Client ID Metadata Documents when OAuth discovery advertises CIMD support, including servers that also expose a dynamic-registration endpoint. This keeps automatic MCP OAuth on the server-advertised path instead of incorrectly asking the user to register an app.

Flow

reserve popup -> probe OAuth discovery
  -> CIMD advertised: create/reuse metadata client -> start OAuth
  -> otherwise DCR advertised: register client -> start OAuth
  -> otherwise: show manual client setup

The probe handoff uses the SDK canonical OAuthProbeResult, so new discovery capabilities cannot be dropped by a duplicated UI-only type.

Validation

• bunx vitest run src/components/add-account-modal.test.ts (45 tests)
• bun run typecheck
• bunx oxlint -c ../../.oxlintrc.jsonc src/components/add-account-modal.tsx src/components/add-account-modal.test.ts --deny-warnings
• ./node_modules/.bin/oxlint -c .oxlintrc.jsonc packages/core/sdk/src/testing/oauth-test-server.ts e2e/selfhost/mcp-oauth-cimd-connect.test.ts --deny-warnings
• ./node_modules/.bin/oxfmt --check packages/core/sdk/src/testing/oauth-test-server.ts e2e/selfhost/mcp-oauth-cimd-connect.test.ts
• ./node_modules/.bin/vitest run --project selfhost selfhost/mcp-oauth-cimd-connect.test.ts — browser E2E passed with recording and trace