fix(encoder): honor default struct tag so JSONOutputFormatParam.Type is never elided

[spor3006]

Fixes #328.

Problem

JSONOutputFormatParam (and the beta variant) declare a constant discriminator that the SDK promises will marshal as "json_schema" even at the zero value:

type JSONOutputFormatParam struct {
    Schema map[string]any      `json:"schema,omitzero" api:"required"`
    // This field can be elided, and will marshal its zero value as "json_schema".
    Type   constant.JSONSchema `json:"type" default:"json_schema"`
    paramObj
}

The default:"json_schema" tag is honored by internal/apijson/encoder.go (used for response decoding paths), but param.MarshalObject — the path that produces request bodies — calls through to the shimmed encoding/json in internal/encoding/json/, which never read the tag. When the discriminator was missing, the Anthropic API silently hung the request rather than 400-ing, and the SDK's 10-minute timeout fired with context deadline exceeded. The Python SDK is unaffected because it serializes the discriminator explicitly.

Fix

Teach the shim struct encoder to honor default:"..." on string-kinded fields. At field-discovery time we pre-marshal the literal once; at encode time, if the field's value is zero, we write the pre-marshaled literal in place of running the field encoder. Non-zero values bypass the default and round-trip unchanged.

This is the smallest fix consistent with the existing tag contract: nothing in the generated code needs to change, the discriminator continues to be elidable at the call site, and the default: tag now means the same thing on both the request and response sides of the SDK.

Tests

• TestDefaultStructTag (packages/param/encoder_test.go) — direct encoder-level coverage: zero plain-string, zero named-string, non-zero preserved, no-default-tag still emits empty. Confirmed to fail on main and pass with the fix.
• TestJSONOutputFormatParamTypeAlwaysMarshaled (messageutil_test.go) — end-to-end regression: the three shapes from the issue (JSONOutputFormatParam alone with zero Type, with explicit Type, and nested inside MessageNewParams).

Notes

• Scoped to reflect.String-kinded fields; that matches the only kind generated code uses default: on today (constant.* named string types) and matches parseDefaultStructTag in internal/apijson/tag.go. Widening later if other kinds need it is mechanical.
• The shim encoder file is forked from the standard library with EDIT(begin)/EDIT(end) markers; this change keeps that convention so the diff against upstream stays auditable.