fix: Model cache toast warning fires on every first message in OpenCode plugin sandbox

[garnetlyx]

Problem

When OmO runs as an OpenCode plugin (Bun runtime sandbox), the "Model Cache Not Found" toast warning appears on every first message of a session, even though the cache was successfully written moments before.

Root Cause

isModelCacheAvailable() calls hasProviderModelsCache(), which ultimately calls existsSync() on the cache file. In the OpenCode plugin sandbox:

1. The config hook runs updateConnectedProvidersCache() which calls writeFileSync — this succeeds in the sandbox virtual filesystem
2. The chat.message hook then calls isModelCacheAvailable() which calls existsSync() — but this runs in a different sandbox context
3. The file written in step 1 is invisible to the filesystem check in step 2
4. Result: isModelCacheAvailable() returns false every time, triggering the warning toast

Key Files

• src/shared/json-file-cache-store.ts — has() uses existsSync() as its final fallback
• src/shared/connected-providers-cache.ts — hasProviderModelsCache() delegates to the stores has()
• src/shared/model-availability.ts — isModelCacheAvailable() triggers the false-negative path

Fix

Added in-process tracking flags so that has() never needs to hit existsSync() if the cache was already written in the current process:

json-file-cache-store.ts

• Added writtenInCurrentProcess module-level boolean flag
• write() sets writtenInCurrentProcess = true on success
• has() checks memoryValue → writtenInCurrentProcess → existsSync() (three-tier)
• resetMemory() resets the flag

connected-providers-cache.ts

• Added providerModelsCacheWrittenInCurrentProcess module-level flag
• writeProviderModelsCache() sets it to true
• hasProviderModelsCache() checks the flag before delegating to the stores has()
• _resetMemCacheForTesting() resets it

This ensures that in sandbox environments where existsSync() is unreliable across hook contexts, the cache is still correctly detected as available.

Impact

• Eliminates spurious "Model Cache Not Found" toast warnings in the OpenCode plugin runtime
• No behavior change for non-sandbox environments (falls back to existsSync() as before)
• All 5450+ existing tests pass