Private add_tool from Tool object instead of function

[pietrozullo]

Add private method to add tools from tools.

Motivation and Context

There should be a way to add a tool without having to specify a function but constructing the tool directly. This can be handy when one has the inputSchema but not the function. Creating a function given a schema programmatically is not trivial.
The function is a private member just for backward compatibility, probably, the original add_tool should have been add_tool_from_fn.

How Has This Been Tested?

I ran tests.

Breaking Changes

No br.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[mcp-shadow]

1. The PR removes the annotations parameter from the add_tool method signature, which is a breaking change despite the author claiming "No br." (no breaking changes). This could break existing code that passes annotations to add_tool.

2. The indentation change in the _add_tool method causes a logical error - the return existing is now only executed when warn_on_duplicate_tools is True, changing the behavior of the method.

3. Private methods should typically be added when there's a clear internal need, but the PR description suggests this is for external usage ("There should be a way to add a tool without..."). If this functionality is needed externally, it should be a proper public API method with tests and documentation.

4. The PR doesn't include tests demonstrating the specific use case where this would be beneficial.