test: make unit-core and unit-connectors required rig groups

Both now run green and standalone (no external services; integration
tests skip cleanly when credentials are absent), so drop optional: true
to make them blocking merge gates per UN-3635.

unit-backend stays optional until the rig provisions a reachable DB_HOST
for it (UN-3636 follow-up).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C5HQX5CSoMR6RzHtXcfwJt

Author: chandrasekharan-zipstack

tests/groups.yaml

@@ -131,7 +131,6 @@ groups:
     paths: [tests]
     uv_sync_group: test
     coverage_source: src
-    optional: true
 
   unit-core:
     tier: unit
@@ -140,7 +139,6 @@ groups:
     # No `test` uv group in unstract/core today; rig still injects pytest plugins.
     install_editable: true
     coverage_source: src
-    optional: true
 
   # ── Integration tier: needs infra but not full platform ────────────────────
   integration-workflow-execution: