fix: misleading JSONPath example and inconsistent message.path descriptions

[sakinaroufid]

Summary

The path property on message_error, message_warning, and message_info is an optional RFC 9535 JSONPath pointer used to anchor a message to a specific component of the response payload. Its JSON Schema description is currently inconsistent across the three sibling schemas, and the example in message_error.json references a field that does not exist in the protocol.

Findings

• message_info.json — description omits an example, giving schema consumers no pattern to follow.
• message_warning.json — phrases the field as "JSONPath (RFC 9535) to related field", diverging from the sibling wording.
• message_error.json — canonical phrasing, but the example points at $.items[1]. No top-level items collection is defined anywhere in checkout.json, cart.json, or order.json; the actual array is line_items (see also the JSONPath examples used in discount.json and message_warning.json). The example is therefore unresolvable against any valid response and is actively misleading.

Change

Unify the three description strings on a single canonical form — "RFC 9535 JSONPath to the component the message refers to (e.g., \$.line_items[0])." — matching the example pattern already used elsewhere in the schema set.

Test plan

• JSON parses cleanly on all three files.
• `cspell` pass on changed files (no new lexemes introduced).
• `@biomejs/biome lint` pass.
• Example JSONPath (`$.line_items[0]`) resolves against the `line_items` array declared in `checkout.json`, `cart.json`, and `order.json`.