fix(mcp_server): split get_or_create_collection on reopen (follow-up to #1262)

#1262 split `get_or_create_collection` into `get_collection` + fallback
`create_collection` inside `ChromaBackend.get_collection`, fixing the
chromadb 1.5.x Rust-binding SIGSEGV that fires when stored collection
metadata differs from the call-site's `_HNSW_BLOAT_GUARD` payload.

The MCP server's `_get_collection(create=True)` carries the same metadata
payload at `mcp_server.py:287` and routes through chromadb's Python
client directly, bypassing the backend layer. Both `tool_add_drawer`
and `tool_diary_write` reach this site on every invocation, and the
Stop hook fires `mempalace_diary_write` at session end — which was
exactly the crash path #1089 named.

Apply the same try/except split here so legacy palaces whose stored
metadata predates the bloat-guard expansion no longer crash on the
MCP-server reopen path. Regression test patches
`get_or_create_collection` at the chromadb client class level (not the
instance — chromadb's mtime-change detection rebuilds the client between
calls, so an instance-level spy doesn't survive) and asserts the second
`_get_collection(create=True)` call never reaches it.

mempalace/mcp_server.py

@@ -57,6 +57,8 @@ from .config import (  # noqa: E402
     sanitize_content,
 )
 from .version import __version__  # noqa: E402
+from chromadb.errors import NotFoundError as _ChromaNotFoundError  # noqa: E402
+
 from .backends.chroma import (  # noqa: E402
     ChromaBackend,
     ChromaCollection,
@@ -284,14 +286,22 @@ def _get_collection(create=False):
             # palaces whose collections were created before this fix (the
             # runtime config does not persist cross-process in chromadb 1.5.x,
             # so the retrofit runs every time _get_collection opens a cache).
-            raw = client.get_or_create_collection(
-                _config.collection_name,
-                metadata={
-                    "hnsw:space": "cosine",
-                    "hnsw:num_threads": 1,
-                    **_HNSW_BLOAT_GUARD,
-                },
-            )
+            #
+            # ChromaDB 1.5.x's Rust binding SIGSEGVs when get_or_create_collection
+            # is called with metadata that differs from what's stored. The split
+            # below skips the metadata-comparison codepath for existing
+            # collections, mirroring the backend-layer fix from #1262.
+            try:
+                raw = client.get_collection(_config.collection_name)
+            except _ChromaNotFoundError:
+                raw = client.create_collection(
+                    _config.collection_name,
+                    metadata={
+                        "hnsw:space": "cosine",
+                        "hnsw:num_threads": 1,
+                        **_HNSW_BLOAT_GUARD,
+                    },
+                )
             _pin_hnsw_threads(raw)
             _collection_cache = ChromaCollection(raw)
             _metadata_cache = None