Fix GraphQL introspection health diagnostics

[davidwrossiter]

Summary

• validate GraphQL credentials with authenticated schema introspection and classify authentication, transport, and schema failures
• expose failed tool-sync health to the web UI and agent-facing connection tools instead of presenting a healthy zero-tool connection
• preserve offline schema snapshots and existing tools during transient live-introspection failures
• add focused protocol, core SDK, and hosted-emulator regression coverage

Testing

• bun run format:check
• bun run lint
• bun run typecheck
• bun run test
• cd e2e && E2E_SELFHOST_URL=http://localhost:45584 bunx vitest run --project selfhost scenarios/graphql-introspection-health.test.ts

Fixes #1473

[davidwrossiter]

UI verification

Before: failed introspection appeared as a generic empty catalogue	After: invalid credential is rejected before creation

Existing failed connection: actionable tool recovery	Account list: matching health verdict

Captured by the hosted-emulator regression scenario included in this PR.

[davidwrossiter]

btw this is related to #1360, I built on the approach there and extended it to cover credential preflight

[RhysSullivan]

Thanks for the PR! Made a few tweaks, confirming the changes then should be good to merge

[davidwrossiter]

Thanks for the PR! Made a few tweaks, confirming the changes then should be good to merge

Lovely