fix(persistence): validate delete and exist keys

[WH-2099]

Description

Validate persistence keys before Delete and Exist reach the cache, storage, or database.
This aligns them with Save and Load and prevents traversal keys from probing or deleting another tenant or plugin’s data.

Refs langgenius/dify#37990

Type of Change

• Bug fix
• New feature
• Refactor
• Performance improvement
• Other

Essential Checklist

Testing

• I have tested the changes locally and confirmed they work as expected
• I have added unit tests where necessary and they pass successfully

Bug Fix (if applicable)

• I have used GitHub syntax to close the related issue (the related issue is a cross-repository umbrella checklist)

Additional Information

• go vet ./internal/core/persistence
• go test ./internal/core/persistence -run ^TestPersistencePathTraversal$ -count=1
• The local persistence suite skips its service-backed assertions when Redis or PostgreSQL is unavailable; CI starts both services and runs the regression test.