add linked systems to connection config get

[adamsachs]

Ticket ENG-2590

Based off #7432

Description Of Changes

Adds a linked_systems field to the connection config GET endpoints (both list and detail), exposing which system(s) are linked to each connection config. This is part of the broader system-integration link effort.

Code Changes

Schema (connection_config.py):

• Added LinkedSystemInfo schema with fides_key and name
• Added linked_systems: List[LinkedSystemInfo] field to ConnectionConfigurationResponse
• Uses an excluded system field + @model_validator(mode="after") to populate linked_systems from the ORM .system relationship during serialization -- no changes to the core ConnectionConfig model
• Changed ConnectionConfigurationResponseWithSystemKey to inherit from ConnectionConfigurationResponse (instead of ResponseBase), so the detail endpoint also returns linked_systems

Route (connection_endpoints.py):

• Added selectinload(ConnectionConfig.system) to the list endpoint query to avoid N+1 queries. Scoped to this endpoint only -- the detail endpoint uses standard lazy loading (single object)

Design docs: Updated section 2.4 of the technical design doc with implementation details and response example

Tests: Updated key assertions in existing tests and added new tests verifying linked_systems is empty for unlinked configs and correctly populated for linked configs (both list and detail endpoints)

Steps to Confirm

1. GET /api/v1/connection -- each item should include linked_systems: [] or linked_systems: [{fides_key: "...", name: "..."}]
2. GET /api/v1/connection/{key} -- should include linked_systems alongside the existing system_key
3. Endpoints that embed ConnectionConfigurationResponse (e.g. webhook responses) also include linked_systems

Confirmed this is populating in our new UI column (Linked system) as expected:

Pre-Merge Checklist

• Issue requirements met
• All CI pipelines succeeded
• CHANGELOG.md updated
  • Updates unreleased work already in Changelog, no new entry necessary
• Confirmed working in UI with updated FE
• Followup issues:
  • No followup issues
• Database migrations:
  • No migrations
• Documentation:
  • No documentation updates required

[greptile-apps]

Greptile Summary

This PR adds a linked_systems field to connection config GET endpoints, exposing which system(s) are linked to each connection config. The implementation properly uses eager loading (selectinload) for list endpoints to avoid N+1 queries, while the detail endpoint appropriately uses lazy loading for the single-object case.

Key changes:

• Added LinkedSystemInfo schema with fides_key and name fields
• Extended ConnectionConfigurationResponse with linked_systems: List[LinkedSystemInfo]
• Implemented _enrich_linked_systems helper to populate the field from ORM relationships
• Added proper eager loading with selectinload(ConnectionConfig.system) in list endpoints
• Updated tests to verify both empty and populated linked_systems scenarios
• Updated documentation with implementation details and response examples

Minor issue:

• Code duplication between connection_endpoints.py and system.py in the _with_linked_systems transformer function (see inline comment)

Confidence Score: 4/5

• This PR is safe to merge with minimal risk
• The implementation is solid with proper eager loading to prevent N+1 queries, comprehensive test coverage for both empty and populated scenarios, and good documentation. The only issue is minor code duplication in the transformer function that could be refactored for better maintainability, but this doesn't affect functionality or safety.
• Pay attention to src/fides/api/api/v1/endpoints/system.py which has duplicated enrichment logic

Important Files Changed

Filename	Overview
src/fides/api/schemas/connection_configuration/connection_config.py	Added LinkedSystemInfo schema and linked_systems field to response schemas. Clean implementation following existing patterns.
src/fides/api/api/v1/endpoints/connection_endpoints.py	Added _enrich_linked_systems helper and transformer for list endpoint with proper eager loading. Detail endpoint uses lazy loading appropriately.
src/fides/api/api/v1/endpoints/system.py	Added linked systems support to system connections endpoint. Duplicates enrichment logic instead of reusing helper function.

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