feat(py): add tool restarts and refactor Interrupts

[huangjeff5]

• refactor Interrupt, now you use raise Interrupt instead of ctx.Interrupt(); this way there is clearer separation and you only pass ctx when needed.
• add define_interrupt.
• add Tool.restart / run_tool_after_restart to support restart flow.
• extend generate loop and executable prompts to support resume/restart.
• Added samples to show usage (probably easiest to start there).

[gemini-code-assist]

Code Review

This pull request refactors the tool interrupt and resume mechanism, replacing the specific ToolInterruptError with a more general Interrupt exception and introducing flat keyword options for resuming generation (resume_respond, resume_restart, and resume_metadata). Key additions include a restart method on the Tool class to facilitate re-running tools with modified inputs, a define_interrupt helper for explicit human-in-the-loop checkpoints, and updated logic in the generation loop to handle pending outputs and resumed contexts. Feedback focuses on maintaining consistent error types by using GenkitError instead of ValueError and fixing Markdown formatting issues in the sample documentation.