fix(llm_fal_ai): validate OpenAPI generation inputs

[moctarjallo]

What changed

• Resolve fal.ai input schemas from named OpenAPI request/response refs, not only components named Input and Output.
• Map generic generation inputs such as image to schema-specific fields like image_url using the resolved schema.
• Validate required generation inputs before calling fal.ai so missing prompt/image fields fail with a clear Odoo error.
• Add tests for named refs, OpenAPI schema fallback input mapping, and required-field validation.

Why

Some fal.ai endpoints expose schemas through named OpenAPI refs and require fields such as prompt plus image_url. Without resolving those refs, assistant calls can send only an attachment or use the wrong field name, producing avoidable provider failures.

Validation

• python3 AST parse for the touched files
• ruff check llm_fal_ai/models/fal_ai_provider.py llm_fal_ai/tests/test_fal_ai_provider.py
• docker compose exec odoo odoo -c /etc/odoo/odoo.conf -d codex_fal_ai_test --init llm_fal_ai --test-enable --test-tags /llm_fal_ai --stop-after-init --http-port=18069 --gevent-port=18072 --db-filter=codex_fal_ai_test