refactor: Remove exposing schema() for NeugDBSession

[zhanglei1949]

Fix #567

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR refactors schema access to stop exposing schema() on NeugDBSession, instead obtaining schema via transactions (and adds ReadTransaction::schema() to support read-mode execution).

Changes:

• Removed NeugDBSession::schema() from the public API and updated server execution to pass txn.schema() into pipeline execution.
• Added ReadTransaction::schema() to support schema access during read transactions.
• Updated transaction tests to fetch label IDs via txn.schema() rather than via session/db schema accessors.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/transaction/test_acid.cc	Updates tests to use txn.schema() for label/edge IDs; adjusts init flows accordingly.
src/transaction/read_transaction.cc	Implements ReadTransaction::schema() for read-mode schema retrieval.
src/server/neug_db_session.cc	Switches query execution to use per-transaction schema access; removes reliance on session schema.
include/neug/transaction/read_transaction.h	Exposes ReadTransaction::schema() in the public header.
include/neug/server/neug_db_session.h	Removes schema() from NeugDBSession interface and drops Schema fwd declaration.

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