fix/mcp-taxonomy-pagination

[Formatted]

## What does this PR do?

Fixes a silent data-loss bug (MemPalace#171) in the four MCP taxonomy tools (mempalace_status, mempalace_list_wings, mempalace_list_rooms, mempalace_get_taxonomy). On palaces with more than ~10k drawers, all four functions called col.get(limit=10000) in a single shot and wrapped the whole thing in except Exception: pass. Two things could go wrong:

1. ChromaDB silently caps or returns None for metadatas on large collections, causing a TypeError: 'NoneType' is not iterable that gets swallowed — result: {"wings": {}} on a 96k-drawer palace.
2. Even when it doesn't raise, a single 10k-item fetch misses everything beyond the cap.

The fix extracts a shared _iter_metadatas(col, where=None) generator that pages through the collection in batches of 500 (the same pattern already used correctly in layers.py), guards against None metadatas at each page, and logs a warning instead of silently swallowing errors. All four taxonomy functions are updated to use it.

## How to test

# Install deps
pip install -e ".[dev]"

# Run the new pagination tests (no network needed — uses in-memory fixtures)
python -m pytest tests/test_mcp_server.py::TestTaxonomyPagination -v

# Full suite
python -m pytest tests/ -v

The three new tests in TestTaxonomyPagination monkey-patch _TAXONOMY_BATCH = 2 to force multi-page fetching on a small fixture, then assert all items are counted across pages. A third test confirms graceful handling when ChromaDB returns None for metadatas.

## Checklist

• Tests pass (python -m pytest tests/ -v)
• No hardcoded paths
• Linter passes (ruff check .)