[codex] Prevent concurrent AP and TP modes

[zhanglei1949]

Summary

Fixes #663.

• Add a NeugDB mode state so embedded AP connections and TP service/session mode cannot be active at the same time.
• Track AP connection lifetime through close callbacks, and release TP mode when NeugDBService is destroyed.
• Stop NeugDBService from silently closing embedded connections on construction; it now rejects active AP connections instead.
• Lock Python connect() / serve() entry points and add Python/C++ coverage for the AP/TP exclusion.

Root Cause

AP embedded Connection and TP NeugDBService could previously be active on the same NeugDB instance. That allowed AP flows such as checkpoint/close cleanup to run while TP WAL/session state still referenced the old checkpoint/WAL directory, creating a data-loss hazard.

Tests

• /opt/homebrew/bin/cmake --build build --target test_db_svc transaction_test -j8
• env MODERN_GRAPH_DATA_DIR=/Users/zhanglei/code/cloud-cli/neug/example_dataset/modern_graph build/tests/unittest/test_db_svc --gtest_filter='NeugDBServiceTest.*'
• build/tests/transaction/transaction_test --gtest_filter='WalReplay*:CompactTransactionTest.*'
• PATH=/opt/homebrew/bin:$PATH PYTHON=/opt/homebrew/opt/python@3.13/bin/python3.13 JOBS=8 make dev from tools/python_bind
• /opt/homebrew/opt/python@3.13/bin/python3.13 -m pytest -s tests/test_tp_service.py -k 'serve_rejects_active_embedded_connection or connect_rejected_while_service_active'
• /opt/homebrew/bin/cmake --build build --target test_connection -j8
• env MODERN_GRAPH_DATA_DIR=/Users/zhanglei/code/cloud-cli/neug/example_dataset/modern_graph build/tests/unittest/test_connection
• git diff --check