fix: support defaults for custom JSON wrappers

[deepakganesh78]

Summary

• keep SchemaProvider defaults in their declared JSON Schema wire format
• construct runtime defaults through the wrapper's existing json.Unmarshaler contract
• preserve schema validation before conversion, so invalid defaults still fail registration

Root cause

Default tags were reflect-converted directly from the schema value to the Go field type. A custom schema can expose a primitive wire type while using a struct wrapper in Go, so the primitive value is not directly convertible to the wrapper.

Tests

• go test . -run '^(TestSchema|TestFeatures)$' -count=1
• go test . -count=1
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.9.0 run

The full Windows suite was also attempted; unrelated platform-sensitive TestCLIShutdown and Fiber adapter tests timed out/failed, while the affected core package and configured lint pass.

Fixes #814