docs(messages): add serialization section with dump/load examples

[Youngho Kim (JadenKim-dev)]

Overview

Adds a "Serialization" section to the messages guide covering how to serialize and deserialize LangChain message objects using dump()/load() (JS) and dumpd()/load() (Python). This addresses a long-standing gap where there was no documentation on persisting messages to a database and restoring them.

Type of change

Type: Update existing documentation

Related issues/PRs

• GitHub issue: docs: Request for improved documentation on serializing and deserializing Langchain messages langchainjs#10472
• Feature PR: feat(core): extend load() to accept objects and add dump() serializer langchainjs#10542

Checklist

• I have read the contributing guidelines, including the language policy
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

[github-actions]

❌ Import check failed

This PR contains imports from langchain_core that should be imported from langchain instead.

Detailed issues

Analyzing diff for import issues...
 Found 1 import issues:

File: src/oss/langchain/messages.mdx
Line: 1813
Issue: Import from langchain.messages instead of langchain_core.messages
Current:   from langchain_core.messages import HumanMessage
Suggested: from langchain.messages import HumanMessage
--------------------------------------------------------------------------------

 Found 1 import issues that need to be fixed.

Why this is a problem

The langchain package re-exports many modules and classes from langchain_core. When possible, imports should use langchain instead of langchain_core for:

• Better user experience (single import source)
• Consistency across documentation
• Reduced cognitive load for users

How to fix

Replace the imports as suggested above. For example:

• ❌ from langchain_core.messages import HumanMessage
• ✅ from langchain.messages import HumanMessage

🤖 Automated check

This check is based on the latest analysis of langchain re-exports from langchain_core.