fix: expandConfigStrings recurse into nested []any slices

[Copilot]

expandConfigStrings did not recurse into []any items that are themselves []any, leaving ${VAR} placeholders unexpanded in array-of-array configs like Casbin roleAssignments.

Changes

• engine.go: Extract []any processing into a dedicated expandConfigSlice helper that handles string, map[string]any, and []any items — enabling expansion at arbitrary nesting depth via mutual recursion with expandConfigStrings.
• engine_secrets_test.go: Add TestExpandConfigStrings_NestedArrayOfArrays covering the []any{[]any{"${VAR}", "literal"}} pattern.

authz:
  type: authz.casbin
  config:
    roleAssignments:
      - ["${OAUTH_CLIENT_ID}", "api_client"]  # now correctly expanded

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[Copilot]

Pull request overview

This PR fixes a bug where expandConfigStrings did not recurse into nested []any slices, leaving ${VAR} placeholders unexpanded in array-of-array configurations like Casbin's roleAssignments.

Changes:

• Refactored slice expansion logic into a dedicated expandConfigSlice helper function that handles string, map, and nested array expansion via mutual recursion
• Added comprehensive test coverage for the array-of-arrays expansion pattern

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
engine.go	Extracted slice processing into expandConfigSlice helper with mutual recursion to support arbitrary nesting depth
engine_secrets_test.go	Added TestExpandConfigStrings_NestedArrayOfArrays to verify expansion in nested array structures