Allow modern Union type for FunctionTool

[ptrxwsmitt]

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Contribution guide.

Describe the bug
Version 1.13.0, 1.14.0

The documentation describes that FunctionTool is capable of using parameters with the modern python Union type i.e. | None (see https://google.github.io/adk-docs/tools/function-tools/#required-parameters).

Yet it doesn't seem to work:

ValueError: Failed to parse the parameter name: str | None = None of function say_hello for automatic function calling. Automatic function calling works best with simpler function signature schema, consider manually parsing your function declaration for function say_hello.

To Reproduce
Use the following tool with an LLMAgent

from google.adk.tools import FunctionTool

def say_hello(name:str|None=None):
    if name:
        return f"Hello {name}!"
    else:
        return "Hello!"
    
def say_hello_tool():
    return FunctionTool(say_hello)

Expected behavior
No ValueError and correct type identification

[jpantsjoha]

I've submitted a documentation PR addressing this discrepancy between docs and implementation:

PR: google/adk-docs#786

The comprehensive type annotation guide:

• Clarifies that | None syntax is NOT currently supported (despite what existing docs imply)
• Documents the working Optional[T] pattern
• Provides migration examples
• Includes Python version compatibility notes

This should prevent the confusion where docs claim modern Union syntax works but the implementation doesn't support it yet.