fix: resolve protocol mismatch and add CLI validation for --observe flag

- Fix critical protocol mismatch: Remove ContextTraceEmitter setup that incorrectly used LangfuseSink (ActionEvent) with ContextEvent
- Add CLI validation: --observe now rejects unsupported values, only accepts 'langfuse' for Phase 1
- Update example: Use TraceEmitter (ActionEvent) instead of ContextTraceEmitter for correct protocol usage
- Add proper try/finally resource management in langfuse_example.py

Addresses all reviewer feedback from CodeRabbit, Copilot, and Gemini regarding:
✅ Protocol contract violations (LangfuseSink/ContextTraceEmitter mismatch)
✅ CLI input validation (typo prevention)
✅ Resource safety (sink cleanup)
✅ Architecture compliance (action-level tracing sufficient for Phase 1)

Co-authored-by: MervinPraison <454862+MervinPraison@users.noreply.github.com>

Author: praisonai-triage-agent[bot]

src/praisonai/praisonai/cli/app.py

@@ -14,23 +14,18 @@
 
 
 def _setup_langfuse_observability(*, verbose: bool = False) -> None:
-    """Set up Langfuse observability by wiring TraceSink to both action and context emitters."""
+    """Set up Langfuse observability by wiring TraceSink to action emitter."""
     try:
         from praisonai.observability.langfuse import LangfuseSink
         from praisonaiagents.trace.protocol import TraceEmitter, set_default_emitter
-        from praisonaiagents.trace.context_events import ContextTraceEmitter, set_context_emitter
 
         # Create LangfuseSink (auto-reads env vars)
         sink = LangfuseSink()
 
-        # Set up action-level trace emitter
+        # Set up action-level trace emitter (sufficient for Phase 1)
         emitter = TraceEmitter(sink=sink, enabled=True)
         set_default_emitter(emitter)
 
-        # Set up context-level trace emitter
-        ctx_emitter = ContextTraceEmitter(sink=sink, enabled=True)
-        set_context_emitter(ctx_emitter)
-        
     except ImportError:
         # Gracefully degrade if Langfuse not installed
         pass
@@ -151,8 +146,10 @@ def main_callback(
     if json_output:
         state.output_format = OutputFormat.json
 
-    # Set up observability if requested
-    if observe == "langfuse":
+    # Validate and set up observability if requested
+    if observe:
+        if observe != "langfuse":
+            raise typer.BadParameter(f"Unsupported observe provider: {observe}")
         _setup_langfuse_observability(verbose=verbose)
 
     # Determine output mode