fix: ship libmariadb3 in runtime image and clarify missing-system-library error (#988)

[Bl3f]

Summary

Fixes #988 — the getnao/nao Docker image bundled mysqlclient (built against MariaDB Connector/C) but not its runtime shared library libmariadb.so.3, so any MySQL datasource failed at import time with a misleading "package required" message.

Two changes:

1. Dockerfile — install libmariadb3 in the runtime stage so import MySQLdb (and thus the ibis MySQL backend) works out of the box. The builder stage already installs libmariadb-dev; only the runtime lib was missing.
2. cli/nao_core/deps.py — require_database_backend / require_dependency previously caught any ImportError and reported pip install 'nao-core[mysql]', even when the real cause was a missing native library. They now distinguish the two cases:
   • ModuleNotFoundError → the Python package is genuinely missing → existing MissingDependencyError (pip/uv install hint).
   • Plain ImportError from a failed shared-object load → new MissingSystemLibraryError that surfaces the real loader error (e.g. libmariadb.so.3: cannot open shared object file) and suggests installing the system library.
   • Any other ImportError is re-raised unchanged instead of being masked.

Root cause

$ python -c "import ibis.backends.mysql"
ImportError: libmariadb.so.3: cannot open shared object file: No such file or directory

ibis/backends/mysql/__init__.py imports MySQLdb at module load; MySQLdb._mysql links libmariadb.so.3. With the lib absent, the import raises ImportError, which require_database_backend("mysql") masked behind the "package required" message.

Testing

• ✅ uv run pytest tests/nao_core/test_deps.py — 5 passed (incl. new system-library / unrelated-error cases)
• ✅ uv run pytest tests/nao_core/commands/sync/test_database_provider.py — 12 passed
• ✅ make lint (ty, ruff check, ruff format) — all checks passed
• ✅ End-to-end repro: hid /lib/x86_64-linux-gnu/libmariadb.so.3 and confirmed the new message; restoring the lib makes require_database_backend("mysql") succeed.
• ✅ Confirmed libmariadb3 provides /usr/lib/x86_64-linux-gnu/libmariadb.so.3.

Before vs. after error message:
mysql_error_message_before_after.txt

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