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/agent/tools/mcp.py

@@ -616,7 +616,7 @@ async def connect_mcp_servers(
         try:
             result = await connect_single_server(name, cfg)
         except Exception as e:
-            logger.error("MCP server '{}' connection failed: {}", name, e)
+            logger.exception("MCP server '{}' connection failed: {}", name, e)
             continue
         if result is not None and result[1] is not None:
             server_stacks[result[0]] = result[1]