[IDEA-386673] MCP structured output error responses incorrectly validated against success schema#3439
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When a tool with a typed return value (structured output enabled) throws a
McpExpectedError, the error's mcpErrorStructureContent is forwarded into
CallToolResult. The MCP SDK then validates this payload against the tool's
success output schema, causing -32602 even though the response is an error.
For example, noSuitableProjectError() returns {projects:[...]} as structured
content, but a tool returning BreakpointListResult expects {breakpoints, total}.
The same issue affects any typed tool (e.g. get_all_open_file_paths) when
project resolution fails.
Fix: only populate structuredContent when isError=false, so error responses
are never validated against the success output schema.