[BUG]: Migrate workflow_concurrent_research_writer to the latest Google ADK Workflow API

[KUSHAGRA13]

Name of sample affected

python/agents/workflow_concurrent_research_writer

Description of issue

The workflow_concurrent_research_writer sample is based on an older version of Google ADK and no longer works correctly with the latest ADK releases.

This sample still relies on APIs such as ParallelAgent and ParallelWorker, which have been deprecated in favor of the newer Workflow API. As a result, users following the current documentation encounter multiple issues when trying to run the sample.

This MR updates the sample to align with the latest ADK APIs and includes the following changes:

• Migrates the sample from the deprecated parallel APIs to the Workflow API.
• Updates the implementation to be compatible with the latest google-adk release.
• Fixes the sample package name by removing the invalid hyphen (-), allowing it to be imported as a valid Python package.
• Resolves additional issues discovered while testing the sample against the latest ADK documentation.

Environment

• Tested locally
• Python 3.12
• google-adk 2.4.0

Reproduction steps

While following the official ADK documentation and attempting to run the sample, several issues were encountered due to API changes introduced in recent ADK releases.

References:

• https://adk.dev/graphs/
• https://github.com/google/adk-python

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