fix: make JsonSchemaGenerator thread-safe with caching and synchronized generation (GH-6207)

[suryateja-g13]

Summary

Fixes #6207 — concurrent calls to JsonSchemaGenerator.generateForType() / generateForMethodInput() throw ConcurrentModificationException from JsonPropertySorter inside victools' JacksonSchemaModule.

Root cause: The static SchemaGenerator instances (TYPE_SCHEMA_GENERATOR, SUBTYPE_SCHEMA_GENERATOR) are shared across threads but are not thread-safe. JacksonSchemaModule with RESPECT_JSONPROPERTY_ORDER uses a sorted property list that throws ConcurrentModificationException under concurrent access.

Fix — two-layer defense:

1. ConcurrentHashMap caches — one per (Type, Set<SchemaOption>) for generateForType, one per (Method, Set<SchemaOption>) for generateForMethodInput. Once a schema is generated for a type it is served instantly from cache for all subsequent callers, eliminating the bulk of concurrent generator invocations.

2. synchronized blocks around generateSchema() calls — protects the first-time generation of any given type before the result lands in cache (prevents the race for concurrent first-callers for distinct types).

The caching also provides a meaningful performance improvement: BeanOutputConverter is instantiated on every ChatClient.ResponseSpec.entity() call and invokes JsonSchemaGenerator.generateForType() each time — with this fix the expensive victools generation runs only once per type per JVM.

Test plan

• JsonSchemaGeneratorTests.generateForTypeConcurrently — 20 virtual threads concurrently generate the same type, no CME, all results equal
• JsonSchemaGeneratorTests.generateForMethodInputConcurrently — 20 virtual threads concurrently generate the same method schema, no CME, all results equal
• All 36 existing JsonSchemaGeneratorTests continue to pass

[suryateja-g13]

Closing in favor of the earlier PRs #6216 and #6217 that address the same issue. My PR adds caching on top of synchronization, but the maintainers can decide which approach to prefer.