Update limitations for using both search and functioncall tools

[luyiourwong]

Is your feature request related to a problem? Please describe.
When using Gemini 2.0+ APIs with both a search tool and a functioncall tool, the API returns a error: 400 . Multiple tools are supported only when they are all search tools.
This appears to be a limitation of the service itself and cannot be addressed within this library.

For example, if a developer registers a VertexAISearch tool to allow the model to retrieve internal documents (e.g., product manuals or company policies), and at the same time attempts to use a function tool to perform operations like submitting a request or triggering an action, the API will fail with the above error.

Describe the solution you'd like
Please update the relevant methods and documentation to clearly state this limitation, similar to how it was handled in google/adk-docs#56. This would help developers avoid unexpected runtime errors when configuring tools.

Describe alternatives you've considered
A potential workaround would be to implement a parent agent system that delegates to child agents, each handling a non-conflicting subset of tools. This pattern is discussed in google/adk-python#53 and could help manage tool conflicts by separating them across isolated agents.

[nireekshak]

Hi @luyiourwong, Thank you for raising this feature request! At the moment, in Gemini API it's possible to use one tool at a time such as either the search tool or the function call tool. We understand the value of this functionality, and the team is actively working on it. We’ll be sure to provide an update as soon as more information becomes available.