[ai]bugfix: read PostgreSQL chat lobs in transactions

[hutiefang76]

What's changed?

Fixes #4393.

Chat message history queries now run in read-only transactions. PostgreSQL maps the existing @Lob String field to a large object; without a transaction its driver rejects result materialization in auto-commit mode, causing the chat stream and history endpoints to fail.

The fix preserves the existing column type and message data. It scopes only the two derived read queries used by streamChat, getConversation, and getAllConversations.

Validation

• ./mvnw -q -pl hertzbeat-startup -am -Dtest=ChatMessageDaoPostgresqlTransactionTest -Dsurefire.failIfNoSpecifiedTests=false test
  • PostgreSQL 17 Testcontainers regression: passed (1 test)
• ./mvnw -q -pl hertzbeat-ai -am -Dtest=ConversationServiceImplTest -Dsurefire.failIfNoSpecifiedTests=false test
  • passed (9 tests)

Checklist

• I have read the Contributing Guide.
• I have added the necessary unit/integration test.