chore(py): Add Tool type, update generate call signature, remove dynamic-tools

[huangjeff5]

Summary

• Adds a first-class Tool type from define_tool / @ai.tool
• lets generate / prompts take tools as registry names (str) or Tool instances
• Allow custom override for define_tool's input_schema. This is used to support define_interrupt (followup PR). Normally input_schema is inferred from the tool function, but in define_interrupt, the Interrupt tool fn is Any (since it needs to be dynamic), while allowing the user to provide an input schema to constrain the model when it chooses to call the interrupt tool.
• Removed dynamic tools for now. It's not ready.
• This PR is a pre-req for Tool respond/restart and Dynamic Tools features.

[gemini-code-assist]

Code Review

This pull request refactors tool handling within the Genkit AI package, introducing a dedicated Tool class and updating various interfaces to support a Sequence[str | Tool] type for tools. It also removes the dynamic_tool method and its associated sample code. Several critical issues were identified in the review: the resolve_tool function is imported but not defined in the provided changes, and there are multiple instances where a TypedDict (PromptGenerateOptions) is incorrectly accessed using dot notation, which will lead to runtime errors. Furthermore, the newly added unwrap_wrapped_scalar_tool_input_if_needed helper is currently unused and should be integrated into the tool execution wrapper to correctly handle wrapped payloads from model providers like Gemini.