Use NamedValues for round-trip logger overrides

[Copilot]

The round-trip fixture was still proving logger overrides with hard-coded credential values instead of NamedValue references. This updates the integration scenario so extract/publish exercises NamedValue-backed logger credentials end to end, and phase 4 now overrides the NamedValues rather than the logger payloads.

• Round-trip fixture

  • adds explicit source APIM NamedValues for:
    • Application Insights instrumentation key
    • Event Hub name
    • Event Hub connection string
  • rewires the logger resources to reference those values via {{namedValue}} tokens

• Override generation

  • updates tests/integration/all-resource-types/phases/run-phase4-create-overrides.ps1 to populate target-specific values through namedValues
  • keeps the App Insights logger resourceId override, but removes direct credential overrides from loggers

• Expected extract shape

  • extends expected-structure.json for the added NamedValues
  • changes logger expectations from concrete credential values/redacted fields to NamedValue placeholders

• Example

  namedValues:
    src-nv-appinsights-key:
      value: "<target-ai-key>"
    src-nv-eventhub-name:
      value: "tgt-eh-logs"
    src-nv-eventhub-connection-string:
      value: "<target-eh-connection-string>"
  
  loggers:
    src-logger-appinsights:
      resourceId: "<target-ai-resource-id>"