feat(java): add SchemaGenerator compile-time type-to-JSON-Schema utility

[Copilot]

Compile-time schema generation utility that maps javax.lang.model types to JSON Schema represented as Map.of(...) source code literals, for use by the annotation processor (task 4.2 of the tool ergonomics plan).

SchemaGenerator.java

• Operates exclusively with javax.lang.model APIs — no java.lang.reflect
• generateSchemaSource(TypeMirror, Types, Elements) → schema source for a single type
• generateParametersSchemaSource(List<VariableElement>, Types, Elements) → full parameters object schema with properties/required
• Handles all 24 specified type mappings: primitives, boxed types, String, UUID, OffsetDateTime, JsonNode, Object, Optional variants, List/Collection/Set, Map with typed values, arrays, enums (with constant enumeration), records, POJOs, and sealed interfaces (oneOf)

// Example: given a method parameter of type List<String>, produces:
"Map.of(\"type\", \"array\", \"items\", Map.of(\"type\", \"string\"))"

// For a record like `record Person(String name, Optional<String> nick)`, produces:
"Map.of(\"type\", \"object\", \"properties\", Map.of(\"name\", Map.of(\"type\", \"string\"), \"nickname\", Map.of(\"type\", \"string\")), \"required\", List.of(\"name\"))"

SchemaGeneratorTest.java

• Uses compilation-testing approach: compiles in-memory Java source snippets with javax.tools.JavaCompiler and a custom SchemaCapturingProcessor that exercises SchemaGenerator during annotation processing
• Covers all minimum required type mappings plus nested types, sealed interfaces, and generated source validity (re-compiles generated expressions to prove syntactic correctness)