feat(py): add tool support for openai-compat plugin

[timere-nemo]

Add tool support for OpenAI compat plugin:

• detects and extracts tool call information
• looks up the corresponding tool/action via the GenkitRegistry
• validates and executes the tool using the defined input schema
• formats the result into a tool message and appends it to the conversation
• re-queries OpenAI with the tool result included, continuing the flow seamlessly.

(Also includes a fix for the action context)

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)