feat(core): propagate ToolResultBlock metadata through agent events and AG-UI protocol (#2315)

## Description

Tools can return structured execution context through
`ToolResultBlock.metadata`. Previously, this metadata was preserved on
the tool result block itself, but was not exposed on the fine-grained v2
agent tool-result events emitted during ReAct execution.

This PR propagates `ToolResultBlock.getMetadata()` into the
corresponding core tool-result events so event-stream consumers can
access tool-specific context from `ToolResultTextDeltaEvent`,
`ToolResultDataDeltaEvent`, and `ToolResultEndEvent`.

AG-UI event types already support the optional `rawEvent` base field,
but this PR does not change AG-UI’s default wire output and does not add
a `metadata` field to `AguiEvent.ToolCallResult`. Applications that want
to expose event metadata through AG-UI can opt in with a custom
`AguiEventEnricher`.

## Changes Made

### Core events (`agentscope-core`)

- Added Jackson metadata deserialization support to:
  - `ToolResultStartEvent`
  - `ToolResultTextDeltaEvent`
  - `ToolResultDataDeltaEvent`
  - `ToolResultEndEvent`
- Kept backward-compatible constructors for callers that do not provide
metadata.
- Updated `ReActAgent.runToolBatch` to copy `ToolResultBlock.metadata`
into:
  - streamed tool-result text/data delta events
  - non-streaming tool-result text/data delta events
  - final `ToolResultEndEvent`
- Added tests for:
- metadata round-trip serialization/deserialization on tool-result
events
- metadata propagation from a tool result into emitted ReAct agent
events