Allow using spring's object mapper

[worldtiki]

Summary

Introduces Spring Boot auto-configuration for customizable Jackson ObjectMapper instances used throughout Spring AI, addressing JSON parsing issues with LLM-generated content.

Key Changes

• Added new spring-ai-autoconfigure-json-parser module with auto-configuration
• Provides two specialised ObjectMapper beans:
  • jsonParserObjectMapper - Lenient parsing for LLM responses
  • modelOptionsObjectMapper - Configurable parsing for model options

Configuration

New properties available under spring.ai.json.*:

• Control unescaped characters, date formats, empty string handling
• Configure unknown property and empty bean behavior
• Handle enum coercion for API compatibility

I believe this fixes #2494