[vector store alt 6/9] Rename both stores' lookup to get_collection and get_partition (speedkick) - #1641

…quest

The event backend created a session's vector store collection and segment
store partition on the first request that opened the session, so a search
or a write for an unknown session created storage as a side effect, and
the service locator was the only place that knew both stores' create paths.

The owner is the session. Every path that creates a session row runs
through EpisodicMemoryManager._create_session, which inserts the row and,
when the row is new, creates the session's partitions in its segment store
and its vector store (create_episodic_memory_storage); an equivalent
re-create accepts the row and leaves the storage as it is. The request
path binds handles with the stores' lookups and raises
SessionPartitionMissingError when a partition is absent: a session without
its storage is broken, not new. Deleting a session with no open instance
deletes its partitions by key, so a session whose storage was never fully
created can still be deleted. MemMachine.create_session goes through the
manager for the same reason.

The session's vector store collection is created with the system keys and
the project's own `properties_schema`, resolved and merged as the request
path did before.

The semantic manager owns its one collection and creates it, once, at the
storage's first use. With that, nothing calls the stores' open-or-create.

The API is unchanged: the manager's open-or-create still creates a session
a memory request names, now through the same path.

Ported from MemMachine#1622 (4266b09) onto the chain that keeps per-project
filterable properties.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

The simple chatbot example, the TypeScript REST demo and the Dify plugin's
add-memory tool wrote to a project without creating it, relying on the
write to create it. Each now creates its project before its first memory
request and accepts 409 as the project already existing. No behavior
changes for them; they stop depending on a write creating a project.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01ESpWYTmCR7X3bJEpoA8SAn

Adding memories to, or searching, a project that did not exist created it,
with the server's default configuration, without the caller's knowledge.
Now only the create-project request creates a project: a write or a
search opens the session and answers 404 for an unknown project, as the
search endpoint already promised; the manager's open-or-create goes.

Two callers depended on the implicit creation. `org_id` and `project_id`
default to `universal`, so the API promises the project
`universal/universal`; the server creates it, once, at startup, and leaves
one that already exists as it is. The MCP add tool names its own project
and has no create-project counterpart, so it creates the project it writes
to, once, and says so. The API doc strings and the OpenAPI document say
which requests create a project.

A breaking API change on `speedkick`.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01ESpWYTmCR7X3bJEpoA8SAn

Nothing calls them since a session's storage is created with the session:
`open_or_create_collection` and `close_collection` leave the vector store
interface and its four backends, `open_or_create_partition` and
`close_partition` leave the segment store interface and its
implementation, and the two config-mismatch errors that only open-or-create
raised go with them. A store creates on `create_*`, strictly, and looks up
on `open_*`, answering None; create-if-absent is the owner's, where the
key's provenance is known.

Source changes are deletions only. The tests that exercised
open-or-create as a fixture use a test-side create-if-absent instead, and
the tests of its own semantics go.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01ESpWYTmCR7X3bJEpoA8SAn

A lookup that answers None for an absent collection or partition is a
Python get, not an open. Identifiers only, produced by the script below,
plus the two docstring lines that said "opened"; every contract is as it
was.

```sh
set -e
cd "$(git rev-parse --show-toplevel)"
git ls-files -z 'packages/server/*.py' | xargs -0 perl -0pi -e '
  s/\bopen_collection\b/get_collection/g;
  s/\bopen_partition\b/get_partition/g;
'
uv run ruff check --fix --quiet packages/server
uv run ruff format --quiet packages/server
```

The `open -> get` half of MemMachine#1626 (e9783d9); the `collection -> partition`
half is not carried, because its premise is a store that is one
collection.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>