Add backend diagnostic log redactor

[limityan]

Summary

• Add a shared backend diagnostics redaction module in bitfun-services-core with redact_diagnostic_log_text and redact_diagnostic_log_text_with_report.
• Redact sensitive prompt, payload, token, and path-like values line by line while preserving routing metadata, event/tool shape, line count, and a replacement count for large log text.
• Re-export the diagnostics facade through bitfun_core::service so backend callers can use one project-wide redaction implementation without adding a desktop API.

Validation

• cargo test -p bitfun-services-core
• cargo check --workspace
• cargo test --workspace