Bug Description

When HERMES_LANGFUSE_PUBLIC_KEY is set to a placeholder value (e.g., "placeholder" or "test-key"), the Langfuse SDK plugin registers hooks successfully but never emits traces to the Langfuse server. The failure is completely silent — no errors, no warnings, and no indication that observability is broken.

Steps to Reproduce

1. Set HERMES_LANGFUSE_PUBLIC_KEY to a placeholder value (not a real Langfuse API key)
2. Start Hermes Agent with the Langfuse plugin enabled
3. Run any agent session that should emit traces
4. Check Langfuse dashboard — no traces appear
5. Check Hermes logs — no errors or warnings about invalid credentials

Expected Behavior

The plugin should either:

• Validate the API key at startup and fail fast with a clear error message
• Emit a warning when trace submission fails due to invalid credentials
• Provide some indication that observability is not working

Actual Behavior

The plugin registers hooks normally, appears to be working, but silently fails to emit any traces. This creates a false sense of security — users think observability is working when it's completely broken.

Environment

• Hermes Agent version: latest
• Langfuse SDK plugin: enabled
• HERMES_LANGFUSE_PUBLIC_KEY: placeholder value