[Feature]message level forking

[riturajFi]

Why we need this PR?

This should close Issue (#317 )

This PR adds branch-aware session message handling so Acontext can represent and read message forks correctly across the API, Core service, and SDKs. It enables storing a message with an explicit parent, retrieving a single root-to-leaf branch path, and defaulting session reads to the latest branch when a session has forked history.

Describe your solution

• Added parent_id support when storing messages through the Python and TypeScript SDKs.
• Added leaf_id support when reading messages so clients can request a single branch path from root to a chosen leaf message.
• Updated the API session service and repository to resolve branch paths with a recursive parent-chain query.
• Changed the default session read behavior to prefer the latest leaf branch when a session contains forks.
• Updated Core message processing to operate on branch paths instead of whole-session message lists.
• Added branch-aware locking, buffering, and flush logic so concurrent inserts are serialized per branch rather than per session.
• Extended task-context packing so the LLM sees parent linkage and branch metadata.
• Added and updated tests across API, Core, Python SDK, and TypeScript SDK to cover the new branch behavior.

Implementation Tasks

Please ensure your pull request meets the following requirements:

• Add parent_id support to message creation in the Python SDK and TypeScript SDK.
• Add leaf_id support to message retrieval in the Python SDK and TypeScript SDK, including validation for invalid option combinations.
• Implement branch-path lookup in the API repository and use it in session message reads.
• Update Core message ingestion and flush handling to process branch paths instead of whole-session pending queues.
• Propagate parent/branch metadata into task context and prompt packing.
• Add or update tests for API, Core, Python SDK, and TypeScript SDK behavior.

Impact Areas

Which part of Acontext would this feature affect?

• Client SDK (Python)
• Client SDK (TypeScript)
• Core Service
• API Server
• Dashboard
• CLI Tool
• Documentation
• Other: message branching and session history semantics

Checklist

• Open your pull request against the dev branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.

[riturajFi]

@GenerQAQ @claude @gusye1234 I Would like your review on this