fix(workbench): require models when adding providers

[lucas77778]

Summary

• require at least one selected model before manually adding OAuth, catalog API, gateway, or custom endpoint providers
• persist the selected models on every newly created account and automatically probe/select all models returned by LinkCode Gateway
• keep local auto-detected providers unchanged: they already select every model in the harness's curated model set

Linear: CODE-589

Verification

• pnpm check:ci
• pnpm test — 2964 passed, 1 skipped
• provider add-flow tests — 15 passed
• exercised the provider form in the running webview and recorded the empty-selection validation followed by successful model selection and provider creation

Checklist

• pnpm check:ci and pnpm test both pass (plus cargo fmt / clippy / test for Rust changes)
• I ran the affected surface and observed the change working
• If a wire message changed: WIRE_PROTOCOL_VERSION is bumped (not applicable; no wire change)
• New code and assets are my own work, or their origin and license compatibility are noted above
• Docs and comments are updated where behavior changed (not applicable; UI copy and tests updated)

[linear-code]

CODE-589

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