docs(core): document String structuredContent parsing in JsonSchemaValidator

[slachiewicz]

What this change does

Adds the input-conversion contract of the validate method to the
JsonSchemaValidator interface Javadoc.

The method treats a String argument as a serialized JSON document and parses
it with readTree before validation. It converts any other type directly with
valueToTree. Both consequences follow for callers:

• To validate a root-level string, pass quoted JSON text: "\"red\""
  represents the value red.
• A bare string such as red reaches the parser as an unquoted token, and
  parsing fails with a message that mentions the tool JSON Schema, even though
  the schema is valid.

This contract exists identically in both implementations
(mcp-json-jackson2 at line 70, mcp-json-jackson3 at line 69). The interface
Javadoc did not state it before this change.

Why

During output-schema validation testing, a root-level enum check failed in a
way that pointed at the schema parser rather than at caller-side string
handling. Tracing both default validators revealed the undocumented branch.
Documentation-only changes to a shared SPI carry no migration risk, and the
wording mirrors what the code does so future refactors can update one place.

Testing

No production code changed. Before opening this pull request, I verified:

• mcp-json-jackson3: DefaultJsonSchemaValidatorTests passes, 40 of 40 tests
• mcp-json-jackson2: DefaultJsonSchemaValidatorTests passes, 40 of 40 tests
• spring-javaformat:validate passes for the touched module

Notes for reviewers

If you prefer stronger ergonomics over pure documentation (for example, a
dedicated overload or a clearer parse-failure message), this wording still
stands on its own. Follow-up issues can adjust either message or API without
conflict. Known sharp edge, intentionally out of scope here: the
"Error parsing tool JSON Schema" prefix also appears when the content fails
to parse; existing test assertions pin that string in both modules.

[Kehrlann]

Thanks for the docs update.

Fixes #1110