feat: add multipart/form-data support for file uploads

[fdarian]

Resolves #3

Summary

• Auto-detect zod-form-data file fields and set content type to multipart/form-data
• Generate { type: "string", format: "binary" } OpenAPI schema for file fields
• Add safe optionality check to avoid triggering zfd preprocessing during schema generation

Changes

src/utils/zod.ts

• isSchemaOptional() — Checks optionality without triggering zfd preprocessing (which calls safeParse internally)
• instanceofZodFormDataFile() — Detects zfd.file() schemas by matching the pipe(transform -> custom) pattern in Zod v4 (with Zod v3 ZodEffects fallback)
• schemaContainsFileField() — Checks if an object schema contains any file fields

src/generator/schema.ts

• generateFormDataSchema() — Replaces file fields with z.string().meta({ format: 'binary' }) so zod-openapi generates the correct binary format
• getRequestBodyObject() — Auto-detects multipart/form-data when file fields are present

src/generator/paths.ts

• Replaced safeParse(undefined) with isSchemaOptional() to avoid triggering zfd preprocessing

src/types.ts

• Added 'multipart/form-data' to OpenApiContentType

Test plan

• Auto-detects multipart/form-data for zfd.formData() with zfd.file() fields
• Multiple file fields generate correct binary schemas
• Backward compatibility — non-file schemas still use application/json
• Required/optional fields are marked correctly
• All existing tests pass (136/136)

[fdarian]

Tested

1. Drop deployment upload via Web
2. Drop deployment upoad via API