Reject process-local graph backends in multi-process deployments - #1810

Purpose of the change

Prevent deployments with multiple server processes from silently using graph
backends whose schema, index or semantic-set caches are local to one process.

Issue: #1753

Description

• Declare PROCESS concurrency support on Neo4j semantic storage and the Neo4j
  and NebulaGraph declarative graph stores.
• Validate enabled components before starting background services or restoring
  sessions. Enforce the same limit in resource factories and resolved session
  overrides, including AUTO semantic storage's Neo4j fallback.
• Add deployment scope configuration and an environment override. Multiple
  HTTP processes selected by MEMMACHINE_WORKERS require at least HOST scope;
  deployments on multiple hosts declare CLUSTER.
• Preserve single-process use, disabled components and other semantic backends.
  Document the configuration and the affected backends' limits.

Fixes/Closes

Fixes #1753

Type of change

• Bug fix

How Has This Been Tested?

• Ran the complete server/client non-integration suite in sequential groups:
  2406 passed, 4 skipped. Three existing invalid-credential tests time out
  contacting the OpenAI API; all three fail identically on unchanged main.
• Ran Ruff check and format check successfully.
• Re-ran the focused configuration, scope, startup and factory tests on Linux:
  all 92 passed. The POSIX-specific time-zone regression and the 11 local
  resource-manager tests also passed; the same three external API tests
  failed with connection errors.
• Ran the workflow's ty checks for server Python 3.12/3.14 and client/common
  Python 3.10/3.14 successfully.
• Reproduced startup through the real configuration model and MemMachine
  start/stop with external resources replaced by test doubles: unchanged main
  accepts each affected backend in CLUSTER scope; this change refuses all
  three before creating background tasks and preserves PROCESS startup.

Project test commands:

pytest packages/server/server_tests packages/client/client_tests
ruff check
ruff format --check
ty check --project packages/server --python-version 3.12
ty check --project packages/server --python-version 3.14
ty check --project packages/client --python-version 3.10
ty check --project packages/client --python-version 3.14
ty check --project packages/common --python-version 3.10
ty check --project packages/common --python-version 3.14

Checklist

• Added regression tests for backend declarations, startup, overrides,
  disabled components, repeated calls and failure recovery.
• Updated the configuration documentation.