refactor(logging): preserve tracebacks in remaining except blocks

Follow-up to PR #3651:

- Replace logger.error with logger.exception inside except blocks
  so stack traces are no longer lost:
  - providers/transcription.py (5 occurrences)
  - agent/tools/mcp.py (1 occurrence)

- Replace stdlib logging.getLogger with loguru logger in
  providers/openai_compat_provider.py for consistency.

nanobot/providers/openai_compat_provider.py

@@ -24,8 +24,7 @@ if os.environ.get("LANGFUSE_SECRET_KEY") and importlib.util.find_spec("langfuse"
     from langfuse.openai import AsyncOpenAI
 else:
     if os.environ.get("LANGFUSE_SECRET_KEY"):
-        import logging
-        logging.getLogger(__name__).warning(
+        logger.warning(
             "LANGFUSE_SECRET_KEY is set but langfuse is not installed; "
             "install with `pip install langfuse` to enable tracing"
         )