fix(evaluation): construct LongTermMemory with the concrete declarative params - #1703

Purpose of the change

The evaluation entry point crashes at startup: evaluation/utils/agent_utils.py calls LongTermMemoryParams(...) as a constructor, but since #1395 that name is an Annotated discriminated union of DeclarativeBackendParams | EventBackendParams, and calling it raises TypeError: 'types.UnionType' object is not callable on Python 3.12. This picks the backend explicitly, as #1476 proposes.

Description

• init_memmachine_params builds DeclarativeBackendParams directly instead of the union alias. The keyword arguments are unchanged; that class is what the alias resolved to for the declarative backend, and it is what the server's own service_locator constructs.
• The setup now refuses an event long-term memory configuration with a ValueError naming the selected backend, next to the existing "not configured" check. It reads vector_graph_store off the configuration, which only the declarative LongTermMemoryConf carries, so without the check an event configuration would fail later with an AttributeError.

No change to the server packages. The issue's alternative, making the alias callable via a dispatching factory, is not taken: the union is the intended shape after #1395 and the server never calls the alias.

Fixes/Closes

Fixes #1476

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Manual verification
  • ruff check and ruff format --check on the changed file: clean.
  • In the server environment, LongTermMemoryParams(session_id="s") reproduces the reported TypeError; evaluation.utils.agent_utils imports and init_memmachine_params constructs DeclarativeBackendParams.
  • evaluation/ has no test suite and is outside the packages CI type-checks, so no unit test is added.

Checklist

• I have signed the commit(s) within this pull request
• My code follows the style guidelines of this project (See STYLE_GUIDE.md)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have checked my code and corrected any misspellings

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