fix(umodel): surface authoring warnings during validation

[Chloride233]

Summary

Service.Validate now surfaces warning-only authoring hints for UModel elements that are schema-valid but may be less useful for downstream writes, filters, query display, or topology traversal.

The hints are intentionally narrow:

• entity_set without spec.fields gets a schema-guidance warning.
• metric_set metrics without both unit and data_format get a display-formatting warning.
• entity_set_link types prefixed with candidate_ or maybe_ get a provisional-link warning.

These warnings reuse the existing ValidationResult.Warnings field and do not reject model elements.

What stays unchanged

• Valid model elements remain valid.
• No REST, CLI, MCP schema, SDK, model schema, import, or graphstore behavior changes.
• Existing schema validation errors still come from the schema validator.
• Normal relation types such as related_to are not warned.
• Metrics with data_format but no unit are not warned.

Test plan

go test ./internal/umodel -run TestValidate -count=1
go test ./internal/agentgateway -count=1
git diff --check