Add MongoDB-backed ChatMemoryStore implementation

[VasilijeJukic01]

Issue

Closes langchain4j/langchain4j#4830

Change

Ported from the main repository as requested by @dliubarskyi.

This PR introduces MongoDbChatMemoryStore, providing a native MongoDB-backed implementation for chat history persistence. This allows users already utilizing MongoDB in their architecture to store chat memory without needing to introduce a relational database.

Architectural & Design Decisions:

• New Module (langchain4j-community-mongodb): Created a new module inside the chat-memory-stores category. The ChatMemoryStore relies on standard MongoDB collections and works with any MongoDB deployment (local, self-hosted, etc.). Keeping it separate ensures users don't have to pull in heavy vector search dependencies if they just want standard MongoDB chat storage.
• Serialization: Messages are serialized to JSON strings using the core ChatMessageSerializer rather than custom BSON arrays. This guarantees out-of-the-box compatibility with all current and future ChatMessage types (e.g., Tool execution, System messages) without needing to maintain complex BSON codecs.
• TTL Support: Added native support for automatic message expiration via MongoDB TTL indexes (expireAfterSeconds). It includes graceful error handling for MongoCommandException (Error 85) if a user updates the TTL value on an existing index.

Testing:

• Added MongoDbChatMemoryStoreIT using Testcontainers (mongo:6.0).
• Tests cover positive CRUD operations, negative validation cases, and memoryId isolation validation.

General checklist

• There are no breaking changes
• I have added unit and integration tests for my change
• I have manually run all the unit tests in all modules, and they are all green
• I have manually run all integration tests in the module I have added/changed, and they are all green

• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)
• I have added/updated Spring Boot starter(s) (if applicable)

Checklist for adding new maven module

• I have added my new module in the root pom.xml and langchain4j-community-bom/pom.xml

Checklist for adding new embedding store integration

• I have added a {NameOfIntegration}EmbeddingStoreIT that extends from either EmbeddingStoreIT or EmbeddingStoreWithFilteringIT
• I have added a {NameOfIntegration}EmbeddingStoreRemovalIT that extends from EmbeddingStoreWithRemovalIT

Checklist for changing existing embedding store integration

• I have manually verified that the {NameOfIntegration}EmbeddingStore works correctly with the data persisted using the latest released version of LangChain4j