fix(v2): avoid empty catalog during discovery refresh (refs #159)

[yuhp]

Summary

Addresses #159 where model discovery refresh or cache expiration caused first requests to transiently return 404 (Model not found).

Root Cause

refreshFromCurrentConfig() previously invoked await ctx.provider.reload() unconditionally at the beginning of every refresh. In OpenCode V2, provider.reload() immediately causes the host to recalculate provider/model transforms. During normal discovery and cache expiration refreshes, invoking this before the upstream /v1/models HTTP request completes placed the model registry in a transient empty/transition state while the network request was in-flight, leading to a temporary 404 window.

Changes

• Introduced reloadProviderConfig (defaults to false) in refreshFromCurrentConfig():
  • Normal discovery / cache refresh: skips the upfront ctx.provider.reload(). Existing models remain fully available while background network discovery runs, and replaceInventory() atomically materializes newly discovered models once ready.
  • Configuration / Credential events (config.updated, credential.*): explicitly passes reloadProviderConfig = true to ensure the host provider registry is synchronized before discovering models.
• Added unit test in test/v2/plugin.test.ts verifying that provider catalog reloads are deferred until model discovery completes.

Related to #159 (closes #159).