fix: resolve circular $ref in zod-to-json-schema for branded types

[matantsach]

Summary

Fixes #1739

When a branded Zod type (e.g. z.string().brand<"Id">()) is reused across multiple fields, the generated JSON schema contains a self-referencing definition — a definition whose value is just a $ref back to itself. This makes the schema invalid and causes a cryptic SyntaxError: Unexpected end of JSON input when used with zodTextFormat or zodResponseFormat.

Root Cause

parseBrandedDef delegates to parseDef for the inner type but doesn't pass through the forceResolution parameter. When a definition is being force-resolved, the branded parser's inner call hits refs.seen and creates a $ref back to the same definition path — producing the circular reference.

The same structural issue exists in all transparent wrapper parsers (catch, default, promise, readonly) that delegate to parseDef without forwarding forceResolution. Only parseEffectsDef was already doing this correctly.

Changes

• Pass forceResolution through parseBrandedDef, parseCatchDef, parseDefaultDef, parsePromiseDef, and parseReadonlyDef — matching the existing pattern in parseEffectsDef
• Add test reproducing the exact circular $ref from the issue (branded type reused in multiple fields)

Testing

• New test verifies no self-referencing $ref in definitions and that the branded type resolves to {type: "string"}
• All 116 helper/lib tests pass
• Test is Zod v3 only (v4 uses z4.toJSONSchema() directly, not the vendored converter)