Add structured-output module: entity() mapping to records/lists/maps, StructuredOutputValidationAdvisor retries

ChatClient.CallResponseSpec.entity() mapping LLM JSON to a record (TicketTriage, with a real
enum-constrained Priority field), a List<ActionItem>, and a Map<String,Object> -- every case driven
by a hand-written ScriptedChatModel with no live LLM anywhere.

Key findings, all confirmed by disassembling spring-ai-client-chat-2.0.1.jar and spring-ai-model-2.0.1.jar
rather than trusting docs:

- StructuredOutputValidationAdvisor lives in org.springframework.ai.chat.client.advisor, in the same
  spring-ai-client-chat artifact as ToolCallingAdvisor -- unlike the tool-calling module's Tool Search
  Advisor pieces, it needs no separate Maven Central artifact or version pin.
- entity(Class, spec -> spec.validateSchema()) is sugar: DefaultCallResponseSpec.resolveAdvisorChain
  builds a real StructuredOutputValidationAdvisor from the same JSON schema BeanOutputConverter uses
  to parse the response, and pushes it onto the advisor chain for that one call.
- The schema/format instructions are baked into the user message once, up front, by entity() itself,
  before the advisor chain runs at all. A validation retry's only contribution is one appended line:
  "Output JSON validation failed because of: <the real schema-validator error>" -- each retry
  re-augments the ORIGINAL request, not the previous attempt's, so corrections never stack.
- Default maxRepeatAttempts is 3 (4 total attempts); default advisorOrder is 2147481647, near
  Ordered.LOWEST_PRECEDENCE.
- Exhausting every retry does NOT throw -- adviseCall's loop just returns the last (still invalid)
  response to the caller. Plain entity() with no validation, by contrast, throws immediately on the
  same bad JSON, since BeanOutputConverter.convert() is a separate Jackson deserialization step with
  no retry loop of its own. Both behaviors are captured from real runs (output/02, output/06).
- Spring AI 2.0's JSON stack is Jackson 3 (tools.jackson.databind), not classic com.fasterxml.jackson --
  visible directly in every one of this advisor's constructor and field signatures.

Companion module for "Structured Output in Spring AI 2.0: Records, JSON Schema and Self-Correcting
Responses" on ankurm.com.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01FtpJvZfg4nvLvtzgJTDWpB

structured-output/output/01-entity-record-valid.txt

@@ -0,0 +1,29 @@
+model call count: 1
+
+mapped record: TicketTriage[category=billing, priority=HIGH, requiresEscalation=true, suggestedActions=[Refund the duplicate charge, Reply within 4 hours]]
+
+JSON schema BeanOutputConverter generated for TicketTriage:
+{
+  "$schema" : "https://json-schema.org/draft/2020-12/schema",
+  "type" : "object",
+  "properties" : {
+    "category" : {
+      "type" : "string"
+    },
+    "priority" : {
+      "type" : "string",
+      "enum" : [ "LOW", "MEDIUM", "HIGH", "CRITICAL" ]
+    },
+    "requiresEscalation" : {
+      "type" : "boolean"
+    },
+    "suggestedActions" : {
+      "type" : "array",
+      "items" : {
+        "type" : "string"
+      }
+    }
+  },
+  "required" : [ "category", "priority", "requiresEscalation", "suggestedActions" ],
+  "additionalProperties" : false
+}