feat(v3): apply schema default values via generated SetDefaults (#138)

[lerenn]

Issue

Fixes #138 — default values in the specification were parsed into the schema but never used in the generated code.

Change

Object schemas now get a SetDefaults() method when at least one optional (pointer) property declares a scalar default. Calling it fills unset (nil) fields with their default and leaves already-set fields untouched:

func (t *SettingsSchema) SetDefaults() {
    if t.Retries == nil {
        v := int64(3)
        t.Retries = &v
    }
    // ...
}

Supported default types: string, boolean, integer (int32/int64) and number (float32/float64), matching the field type produced by the schema-name template. Date/time, object and array defaults are intentionally out of scope. Two new v3 template helpers (hasScalarDefault, defaultLiteral) drive the generation.

Test

test/v3/issues/138 defines a Settings schema with integer/number/boolean/string defaults; the suite asserts SetDefaults() fills unset fields and preserves set ones. Before the change the method is not generated, so the package fails to compile; after, it passes.

test/v3/issues/224's golden file is regenerated (its margin property gains a SetDefaults). go generate ./... is otherwise clean.

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