fix(coderd/x/chatd): fix flaky TestSpawnComputerUseAgentInheritsContext (#24666)

Fixes flaky `TestSpawnComputerUseAgentInheritsContext`.

- The test inserts an Anthropic provider directly into the DB after
`CreateChat` has already been called
- The server's background goroutine may have already cached the provider
list (OpenAI only) via `configCache.EnabledProviders()` with a 10s TTL
- The direct DB insert bypasses the pubsub event that production uses to
invalidate the cache
- `isAnthropicConfigured()` returns the stale cached result, making
`computer_use` appear unavailable
- Fix: call `server.configCache.InvalidateProviders()` after the insert,
mirroring what production does via pubsub

CI failure:
https://github.com/coder/coder/actions/runs/24829197096/job/72673070101?pr=24648

> 🤖

coderd/x/chatd/subagent_context_internal_test.go

@@ -477,6 +477,12 @@ func TestSpawnComputerUseAgentInheritsContext(t *testing.T) {
 	ctx := chatdTestContext(t)
 	parentChat := createParentChatWithInheritedContext(ctx, t, db, server)
 	insertEnabledAnthropicProvider(ctx, t, db, parentChat.OwnerID)
+	// The direct DB insert above bypasses the pubsub event that
+	// production uses to invalidate the provider cache. Explicitly
+	// invalidate here so the background processing goroutine does
+	// not serve a stale provider list (OpenAI only) that was cached
+	// before the Anthropic provider was inserted.
+	server.configCache.InvalidateProviders()
 
 	tools := server.subagentTools(ctx, func() database.Chat { return parentChat }, parentChat.LastModelConfigID)
 	tool := findToolByName(tools, spawnAgentToolName)