Feature Request: Stream tool execution status and output (e.g., Bash command stdout/stderr)

### Problem

Currently, the streaming API (`includePartialMessages: true`) only provides events for Claude's **response generation** phase:
- `content_block_start` / `content_block_delta` / `content_block_stop` for text and tool call declarations

However, there are **no streaming events during tool execution**. The message flow shows a gap:

StreamEvent (content_block_stop) ← tool call declaration complete
AssistantMessage
... tool executes ... ← NO EVENTS HERE
... next turn ...

This is especially problematic for **long-running Bash commands**, where users have no visibility into:
1. Whether the command has started
2. What output is being produced (stdout/stderr)
3. When the command finishes

### Proposed Solution

Add new `StreamEvent` types for tool execution lifecycle:

| Event Type | Description |
|------------|-------------|
| `tool_execution_start` | Tool begins execution (includes tool name, tool_use_id) |
| `tool_execution_output` | Incremental stdout/stderr from the tool (for Bash, shell output) |
| `tool_execution_end` | Tool execution complete (includes exit code for Bash) |

### Example Expected Flow

StreamEvent (content_block_start) - tool_use block
StreamEvent (content_block_delta) - tool input JSON
StreamEvent (content_block_stop) - tool call declaration complete
StreamEvent (tool_execution_start) - { tool: "Bash", tool_use_id: "..." }
StreamEvent (tool_execution_output) - { chunk: "Installing dependencies...\n" }
StreamEvent (tool_execution_output) - { chunk: "npm WARN deprecated...\n" }
StreamEvent (tool_execution_end) - { exit_code: 0 }
AssistantMessage

### Use Cases

1. **Real-time terminal UI**: Show command output as it streams, similar to a real terminal
2. **Progress indicators**: Display "Running npm install..." with live output
3. **Long-running commands**: Provide feedback during builds, tests, or deployments that may take minutes
4. **Debugging**: See exactly what a command outputs when troubleshooting failures

### Additional Context

This feature would significantly improve the developer experience when building interactive CLI tools or chat interfaces that invoke shell commands. Currently, developers have to either:
- Show a generic "executing..." spinner with no real feedback
- Or implement custom workarounds outside the SDK

Thank you for considering this enhancement!