test(coderd/x/chatd): seed anthropic provider for computer_use tests (#24611)

`TestSubagentLifecycleToolsIncludePersistedSubagentTypeAcrossVariants/ComputerUse`
and two adjacent positive tests passed a static Anthropic key into
`newInternalTestServer`, but `seedInternalChatDeps` only inserts an
OpenAI
provider. At runtime, `Server.resolveUserProviderAPIKeys` calls
`chatprovider.PruneDisabledProviderKeys`, which clears `keys.Anthropic`
because Anthropic is not in the enabled DB provider set, so the
`computer_use` execution path loses its key.

Add a focused test helper `seedEnabledAnthropicProvider` and use it only
in
the positive tests that actually drive a `computer_use` spawn through
the
runtime key-resolution path (the `computer_use` branch of
`TestSubagentLifecycleToolsIncludePersistedSubagentTypeAcrossVariants`,
`TestSpawnAgent_ComputerUseUsesComputerUseModelNotParent`, and
`TestSpawnAgent_ComputerUseInheritsMCPServerIDs`).
`seedInternalChatDeps`
stays unchanged, so the negative availability tests continue to model
the
"Anthropic unavailable" fixture. No production code is modified.

Closes https://github.com/coder/internal/issues/1486

> This PR was opened by Mux working on Mike's behalf.

coderd/x/chatd/subagent_context_internal_test.go

@@ -472,12 +472,11 @@ func TestSpawnComputerUseAgentInheritsContext(t *testing.T) {
 
 	db, ps := dbtestutil.NewDB(t)
 	require.NoError(t, db.UpsertChatDesktopEnabled(chatdTestContext(t), true))
-	server := newInternalTestServer(t, db, ps, chatprovider.ProviderAPIKeys{
-		Anthropic: "test-anthropic-key",
-	})
+	server := newInternalTestServer(t, db, ps, chatprovider.ProviderAPIKeys{})
 
 	ctx := chatdTestContext(t)
 	parentChat := createParentChatWithInheritedContext(ctx, t, db, server)
+	insertEnabledAnthropicProvider(ctx, t, db, parentChat.OwnerID)
 
 	tools := server.subagentTools(ctx, func() database.Chat { return parentChat }, parentChat.LastModelConfigID)
 	tool := findToolByName(tools, spawnAgentToolName)