feat(workflow): Allow ToolNode to accept JSON string or Content inputs

[baptmont]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: N/A
• Related: N/A

2. Or, if no issue exists, describe the change:

Problem:
Previously, ToolNode in ADK workflows required input to be strictly a Python dict or None. This limited workflow flexibility, preventing users from chaining a ToolNode directly downstream of nodes (like LLM Agents or custom function nodes) that output inputs as serialized JSON strings or types.Content objects without manual parsing boilerplate.

Solution:
Enhanced ToolNode._run_impl in _tool_node.py to dynamically coerce input formats before tool execution:

• Extract text from types.Content when received.
• Strip and deserialize JSON strings via json.loads if received.
• Coerce None to {}.
• Keep strict validation raising TypeError for non-dictionary representations (e.g. lists, invalid JSON).

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Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Pytest Summary:

tests/unittests/workflow/test_tool_node.py .......                       [100%]
======================== 7 passed, 4 warnings in 2.80s =========================

Manual End-to-End (E2E) Tests:
Verified all workflow tests pass locally with no regressions:

uv run pytest tests/unittests/workflow/
...
==== 600 passed, 10 skipped, 26 xfailed, 9 xpassed, 237 warnings in 15.17s =====

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Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

[adk-bot]

Response from ADK Triaging Agent

Hello @baptmont, thank you for contributing this PR! This looks like a great enhancement to ToolNode to handle more flexible input formats in workflows.

I noticed that the automated pre-commit check has failed for this pull request. To ensure code formatting and style guidelines are met, could you please run pre-commit locally to format and check your changes?

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pre-commit run --all-files

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