Logging: ADK should avoid logging UGC/PII

### Is your feature request related to a specific problem?
ADK should avoid logging UGC/PII, in particular, in core components that is not opt-in or configurable by consumers.

For example:
https://github.com/google/adk-java/blob/1a3dd612217a05e2f8fff69720087ed1136a09ab/core/src/main/java/com/google/adk/flows/llmflows/BaseLlmFlow.java#L751

### Describe the Solution You'd Like
ADK to remove logging or redact sensitive user generated content, and can ideally apply guardrails to enforce this.

### Impact on your work
Need to actively apply configuration to filter in/out logs which may contain UGC/PII

### Willingness to contribute
Are you interested in implementing this feature yourself or submitting a PR?
Yes.

---

## 🟡 Recommended Information

### Describe Alternatives You've Considered
Workaround exist by adding logback configuration
```

```