Keep parse warnings when a schema is attached

[holodorum]

Keep parse warnings when a schema is attached

SchemaValidator.validate discarded parse-phase warnings, such as duplicate keys, whenever the document had no hard errors: it returned the parse messages only on the error path, and otherwise built a fresh sink holding schema violations alone.

Every consumer was affected: the LSP, the CLI's --schema check, and the Python, Rust, JS and Java bindings. It showed most in the editor, which relies solely on validate when a schema is attached, so schema-backed documents reported no warnings at all.

The fix

validate now returns the parse messages followed by the schema violations. Parse errors still short-circuit, because an unparseable document cannot be checked against a schema.

CLI behaviour

Every schema violation is a warning, so a CLI caller cannot tell one from a parse warning. The check is generalised rather than filtered:

• format, json and yaml with --schema report every diagnostic on stderr and still produce output, failing only on parse errors or, with the new --strict flag, on any diagnostic.
• Without --schema nothing changes unless --strict is given.
• validate keeps failing on any message.

Tests

Covered at the lib, LSP tooling and CLI layers. The VS Code test gives one document both a duplicate key and a missing required property, so a pass proves the schema was attached rather than the document having silently fallen back to the schema-less path, where parse warnings were never dropped.