Improve Gemini tool calling handling

[nmaguiar]

Motivation

• Gemini 2.5/3 can return tool calls as functionCall parts and sometimes encodes args as JSON strings, so the tool-calling loop must handle both shapes.
• Tool declarations may be passed as names or objects, so normalization is required before sending to the API.
• Avoid duplicating or corrupting the conversation state when inserting tool call and response parts.

Description

• Updated js/owrap.ai.js to normalize aTools (accept arrays of tool names) and resolve them to declared tool objects before sending functionDeclarations to Gemini.
• Cleaned tool schemas by removing fn and $id/$schema entries via traverse before including them in body.tools to prevent invalid schema errors.
• Adjusted Gemini tool-calling flow to detect functionCall parts, parse args when they are JSON strings, execute the registered tool function, and append a functionResponse part on a follow-up user message.
• Fixed recursion behavior to avoid re-processing appended parts by invoking rawPrompt with an empty prompt when continuing the conversation, and added inline comments explaining Gemini 2.5/3 tool-calling nuances.
• Updated docs/ow-ai-gpttypes.md with a Gemini 2.5/3 notes section explaining functionCall/functionResponse flow and JSON response schema guidance.

Testing

• No automated tests were executed as part of this change.
• Manual validation steps were not recorded in the commit.
• Documentation was updated to reflect the new behavior in docs/ow-ai-gpttypes.md.
• No breaking changes are expected for existing providers.