fix(tool): return suspended results for external tools (#1668)

## Description

Fixes #1582.

External tools were short-circuited with `Mono.error(new
ToolSuspendException())` before the executor's `ToolSuspendException`
conversion path, so batch execution could convert external tool calls
into generic error results.

This PR:

- Returns `ToolResultBlock.suspended(toolCall)` directly for external
tools in `ToolExecutor`
- Emits `RequireExternalExecutionEvent` when the agent event stream sees
suspended tool calls
- Updates external-tool Javadocs to describe the direct suspended-result
flow
- Adds executor and ReActAgent regressions for `TOOL_SUSPENDED`,
`RUNNING` state, and external execution events