feat(BA-5590): add paginated REST v2 project scoped search endpoints

[hhoikoo]

resolves #10785 (BA-5590)

Add two new paginated REST v2 endpoints for project-scoped search:

• POST /v2/projects/domains/{domain_name}/search -- search projects within a domain
• POST /v2/projects/users/{user_id}/search -- search projects a user belongs to

Both endpoints accept offset-based pagination (limit/offset), filters, and ordering via SearchProjectsRequest. The implementation adds:

• ProjectAdapter.domain_search() and ProjectAdapter.user_search() adapter methods, plus a shared _build_search_querier() helper that converts the request DTO into a BatchQuerier with OffsetPagination.
• V2ProjectHandler.domain_search() and V2ProjectHandler.user_search() REST handler methods.
• Route registrations in the project registry (both guarded by auth_required).

The underlying processor actions (SearchProjectsByDomainAction, SearchProjectsByUserAction) already exist on the epic branch; this PR wires them to the REST v2 surface.

Checklist: (if applicable)

• Mention to the original issue
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation

[Copilot]

Pull request overview

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Adds REST v2 handlers and routes for paginated “scoped” project search, enabling clients to search projects within a domain or within a user’s memberships.

Changes:

• Register two new POST routes under /v2/projects for domain-scoped and user-scoped search.
• Add REST handlers that forward requests to the project adapter.
• Implement adapter methods that convert request DTOs into offset pagination queries and return paginated payloads.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 8 comments.

File	Description
src/ai/backend/manager/api/rest/v2/project/registry.py	Registers the new scoped search endpoints and wires them to handlers.
src/ai/backend/manager/api/rest/v2/project/handler.py	Adds REST handlers for domain/user scoped project search.
src/ai/backend/manager/api/adapters/project.py	Implements adapter-side scoped search logic and pagination payload mapping.
changes/10787.feature.md	Adds changelog entry for the new paginated scoped search endpoints.

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[hhoikoo]

redundant. bridge holds superadmin credentials anyways.