Feature/agentchat message id field 6317 (#6645)

## Why are these changes needed?

This PR implements unique ID fields for AgentChat messages to enable
proper correlation between streaming chunks and completed messages.
Currently, there's no way to correlate `ModelClientStreamingChunkEvent`
chunks with their eventual completed message, which can lead to
duplicate message display in streaming scenarios.

The implementation adds:
- `id: str` field to `BaseChatMessage` with automatic UUID generation
- `id: str` field to `BaseAgentEvent` with automatic UUID generation  
- `full_message_id: str | None` field to
`ModelClientStreamingChunkEvent` for chunk-to-message correlation

This allows consumers of the streaming API to avoid double-printing
messages by correlating chunks with their final complete message.

## Related issue number

Closes #6317

## Checks

- [x] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [x] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [x] I've made sure all auto checks have passed.

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Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>