Remove ineffective agent factory checks and copied session helper mocks

The agent plugin factory tests repeat shape/default assertions that do not exercise the configuration they claim to validate: nonempty arrays, schema keys, routes and plugin names. The session utility suite similarly tests reexport identity/provider names without reading session state, and repeats omitted/undefined defaults and an already-covered filename assertion. Session bridge tests replace core parsing/building with copied implementations, so collaborator drift can pass unnoticed.

Remove these ineffective or duplicate checks, retain the persisted session-path compatibility cases, and run the existing room/provider routing cases with real core helpers. Preserve factory lifecycle, service registration and promoted-memory action behavior. No production changes or new fixture framework.

Acceptance: smaller maintained test code, no loss of unique tested outcomes, all retained factory/provider tests passing with package type/lint and root verification. Registration tests remain until actual consumer coverage can justify their consolidation; this issue does not authorize broad service-coverage removal.