chore: add compile-time interface assertions for tool implementations

[merkulovlad]

Description of Change

Problem:
Currently, explicit compile-time checks exist only for the MCP tool via RequestProcessor and FunctionTool. Other tool
types rely on runtime validation in base_flow, which makes interface conformance checks less explicit.

Solution:
Add compile-time var _ interface assertions to all tool implementations that implement toolinternal.RequestProcessor
and/or toolinternal.FunctionTool, consistent with the existing pattern in mcpTool.

Testing Plan

No tests needed.
This PR only adds explicit compile-time interface checks and does not change runtime behavior.

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.

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[gemini-code-assist]

Code Review

This pull request adds compile-time interface satisfaction checks for various tool implementations, ensuring they adhere to the toolinternal.FunctionTool and toolinternal.RequestProcessor interfaces. I have no feedback to provide.