refactor: Disable auto checkpoint for read-only mode

[zhanglei1949]

When the database is opened in read-only mode, there is no need for us to auto checkpoint on close.

Fix #239
Fix #240

[greptile-apps]

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[Copilot]

Pull request overview

Refactors database shutdown behavior so that automatic checkpoint creation is skipped when the database is opened in read-only mode, avoiding unnecessary persistence work (and potential writes) on close.

Changes:

• Gate createCheckpoint() on DBMode::READ_WRITE in NeugDB::Close(), and add a VLOG entry when checkpointing occurs.
• Remove unused _move_data helper templates from id_indexer.h.
• Adjust closed_ stores in NeugDB::Close() (removes one redundant store; adds another at the end).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/main/neug_db.cc	Skip checkpoint-on-close for read-only mode; add logging; tweak closed_ updates.
include/neug/utils/id_indexer.h	Remove unused helper templates at end of header.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

Comments suppressed due to low confidence (1)

src/server/neug_db_session.cc:108

• The term 'read-only mode' is used for both the database configuration (DBMode::READ_ONLY) and the session access mode (AccessMode::kRead). The second message ('Read-only mode does not support...') can be confused with the DB-level message above. Consider clarifying the access-mode message (e.g., 'Access mode READ does not support write operations') to make client-facing errors unambiguous.

      return neug::Status(
          neug::StatusCode::ERR_INVALID_ARGUMENT,
          "Database is in read-only mode; write operations are not allowed.");
    }
  }
  if (mode == neug::AccessMode::kRead) {
    if (!is_read_only(flags)) {
      return neug::Status(neug::StatusCode::ERR_INVALID_ARGUMENT,
                          "Read-only mode does not support write operations.");

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