Expose ImageV2.installedAgents connection field in v2 GQL and REST (Valkey-backed)

Add an installedAgents connection field to the v2 image type, following the Deployment.replicas scoped-search pattern (gql/deployment/types/deployment.py). The resolver injects the installation scope as a base condition and calls the agent scoped search, so filter / orderBy / pagination of AgentV2Connection work as usual.

Scope:
- Extend the agent adapter admin_search (api/adapters/agent/adapter.py) with a base_conditions parameter, replicating the image-alias adapter pattern.
- Data source at this stage stays Valkey: read the agent id set for the image and pass AgentConditions.by_ids as the base condition (an empty list safely yields no results). Nested filtering by installation is out of scope (added after DB migration).
- Add a scope input DTO (image_id) under common/dto/manager/v2/agent/request.py; node response DTOs unchanged.
- REST v2 scoped search endpoint: POST /v2/agents/images/{image_id}/search.
- hide_agents policy: for non-superadmin with hide_agents enabled, return null for the field (authorization handled at the service/adapter layer).

Success Criteria
- [ ] ImageV2.installedAgents returns a connection of agents that have the image, with working filter / orderBy / pagination
- [ ] installedAgents(first: 0) exposes a total count usable as an installed indicator
- [ ] image with no installed agents: connection with count 0, not an error
- [ ] non-superadmin with hide_agents config enabled receives null for the field
- [ ] REST v2 scoped agent search by image id returns the same data
- [ ] pants test passes for affected packages

JIRA Issue: BA-7161

Start with gql/deployment/types/deployment.py and the agent adapter at api/adapters/agent/adapter.py, then inspect common/dto/manager/v2/agent/request.py and the affected v2 agent routes. Run the affected packages' pants tests. Done means ImageV2.installedAgents and POST /v2/agents/images/{image_id}/search support filtering, ordering, pagination, empty results, counts, and the stated hide_agents behavior.

Written by the indexing model from the issue text.