feat(client): add getSchema to fetch endpoint JSON schemas

[tombeckenham]

Summary

• Adds getSchema(endpointId) method to fetch input/output JSON schemas for any fal endpoint
• Schemas are fully resolved with all $ref references inlined, making them suitable for JSON Schema validators or LLM tool definitions
• Handles circular references by collecting them into $defs and converting circular $ref paths to #/$defs/Name format

Benefits

• LLM tool definitions — Pass resolved schemas directly to LLMs as tool/function definitions, enabling AI agents to call fal endpoints with proper parameter descriptions
• Input validation — Validate requests locally before sending, catching errors earlier with better messages
• Dynamic form generation — Build UIs that automatically generate input forms based on endpoint schemas
• Discoverability — Programmatically explore what any endpoint accepts and returns

Usage

const { input, output } = await fal.getSchema("fal-ai/kling-video/v3/pro/image-to-video");
// input and output are fully resolved JSON schemas

Test plan

• Unit tests cover schema resolution including circular dependencies
• Manual test with fal.getSchema("fal-ai/kling-video/v3/pro/image-to-video") returns valid input/output schemas

Fixes #189

[tombeckenham]

@drochetti what do you think of this approach to get schemas?