Generates function body AST for OpenAPI operations.

This module is primarily exercised via unit tests and mirrors the high‑level behaviour documented in docs/parsingv2.md:

• Replaces path parameters in the URL using String.replace/3
• Builds query strings from opts using URI.encode_www_form/1
• Collects body parameters from the non‑opts function arguments
• Delegates the HTTP call to ExOpenAI.Config.http_client().api_call/6

In the current v2 codegen the real, production function bodies are generated inline in PathModuleGenerator. generate_body/3 exists as a focused unit under test that captures the same ideas without being wired into the runtime generator.