Emit warnings when unknown schemas are mapped to JsonElement

[halotukozak]

Description

When SpecParser encounters a schema it cannot resolve, it maps it to TypeRef.Unknown, which becomes JsonElement in generated code. This happens silently — users have no way to know that part of their API lost type safety. The parser should emit a warning for each TypeRef.Unknown resolution so users can fix their spec or be aware of the limitation.

Acceptance criteria

• SpecParser emits a Warning when a schema resolves to TypeRef.Unknown
• Warning message includes the context (schema name, property name, or endpoint) where the unknown type was encountered
• Warnings are surfaced in ParseResult.warnings
• Tests verify warnings are emitted for unresolvable schemas