Use native tool-calling support in GPT and Claude

GPT and Claude now support passing tool descriptions natively as part of completion requests. Replace our custom (“homegrown”) tool-calling logic with their native tool mechanisms.

- The TLA+ MCP server isn’t publicly accessible, so we can’t use MCP directly.
Instead, translate our MCP server’s tool descriptions into native tool definitions.
- Remove all truncation and pruning of previous messages.

Benefits:
* Better LLM performance: Native tool calling is optimized by the LLM providers
* Better reliability: Native tool calling is more robust than custom JSON parsing
* Cleaner code: Removed ~100 lines of JSON parsing and error handling

Author: lemmy