fix(server): await models before listing - #41783

What

Make the first model.list request against a newly started server wait for model-providing plugins to finish initializing. This restores opencode models --standalone output on cold start instead of silently returning no models.

Closes #41071

Before / After

Before: A standalone server became HTTP-ready while PluginSupervisor was still activating the initial plugin generation. An immediate model.list request read the empty Catalog, returned HTTP 200 with data: [], and the CLI printed nothing before exiting 0.

After: model.list waits for PluginSupervisor.flush before reading the catalog, matching the existing model.default readiness boundary. If initialization does not settle within five seconds, the endpoint returns the existing model-catalog service-unavailable error rather than a successful empty response.

How

• packages/server/src/handlers/model.ts applies the existing bounded model-catalog initialization wait to model.list.
• packages/server/test/model.test.ts starts a fresh server with a configured custom model and immediately requests /api/model, asserting that the cold-start response includes that model.

Scope

• No CLI output or exit-code behavior changes are needed; the fix is at the server model-list consistency boundary.
• No protocol or generated-client surfaces change.
• Provider loading and plugin supervision behavior are unchanged.

Testing

• bun run test test/model.test.ts --rerun-each 3 from packages/server (3 passes)
• bun run test from packages/server (18 passes, 11 skips)
• bun typecheck from packages/server
• Push hook: bun turbo typecheck --concurrency=3 (33 successful tasks)
• bunx prettier --check src/handlers/model.ts test/model.test.ts from packages/server

The regression test was also run before the handler change and reproduced the bug as HTTP 200 with { "data": [] }.