feat: isDelta property in Message for streaming

[acensia]

Description

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Fixes a2aproject/A2A#822 🦕

[gemini-code-assist]

Summary of Changes

Hello @acensia, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces core support for 'message deltas,' enabling the system to handle incremental updates to messages. This feature is crucial for scenarios like streaming responses, where message content is built up and delivered in chunks rather than as a single, complete unit, improving the user experience for long-running or dynamic outputs.

Highlights

• Message Type Enhancement: The Message data model now includes an isDelta boolean field to explicitly mark messages as incremental updates, enabling the system to differentiate between complete messages and partial, streaming updates.
• Delta Message Accumulation Logic: The TaskManager's save_task_event method has been updated to intelligently handle delta messages. If a new message is a delta and shares the messageId with the last message in the task's history, its parts are appended to the existing message's parts, effectively building up the complete message over time.
• Event Consumption Behavior: The EventConsumer's consume_all method now considers a Message event as 'final' only if it is not a delta message. This ensures that streaming delta messages do not prematurely terminate event processing, allowing for continuous updates.
• Default Message Creation: New agent messages generated by TaskUpdater are now explicitly initialized with isDelta=False to maintain expected behavior for non-delta messages, ensuring that only intentionally marked messages are treated as deltas.

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[gemini-code-assist]

Code Review

This pull request introduces support for message deltas. A potential IndexError has been identified in src/a2a/server/tasks/task_manager.py when handling delta messages and an empty history. A code suggestion has been provided to address this.