Allow callers to disable all tools for a single Agent call

[fwang2002]

Description

Problem

Callers cannot disable tool use for a single Generate / Stream call without going through a PrepareStep callback.

In prepareTools (agent.go:987-1024):

for _, tool := range tools {
    if len(activeTools) > 0 && !slices.Contains(activeTools, tool.Info().Name) {
        continue
    }
    ...
}

ActiveTools == nil and ActiveTools == []string{} both mean "include every tool" — this is documented by the test Empty ActiveTools means all tools (agent_test.go:1333). So there is no value of ActiveTools that means
"no tools for this call".

The only existing way to remove all tools for a step is PrepareStepResult.DisableAllTools, which is only reachable from inside a PrepareStep callback. A caller who simply wants to ask the model a tool-free follow-up
question on an already-configured agent has to write a PrepareStep function just to flip a bool.

## Proposal

Add DisableAllTools bool to AgentCall and AgentStreamCall, mirroring the existing PrepareStepResult.DisableAllTools field.