feat(mcp): detect OAuth-protected remote MCP servers (#2363)

* feat(mcp): surface OAuth-required server tests

* fix(mcp): show connection failure details in status cards

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Co-authored-by: RockChinQ <rockchinq@gmail.com>

src/langbot/pkg/provider/tools/loaders/mcp_stdio.py

@@ -52,6 +52,7 @@ class MCPSessionErrorPhase(enum.Enum):
     MCP_INIT = 'mcp_init'
     RUNTIME = 'runtime'
     TOOL_CALL = 'tool_call'
+    OAUTH_REQUIRED = 'oauth_required'
     # Stdio MCP refused because Box is disabled in config or currently
     # unavailable. Not transient — retries would be pointless. The frontend
     # uses this phase to render a localized actionable message instead of