MCP config loading now reports a 'present' flag and distinguishes error types

What

Loading an MCP (Model Context Protocol) config file was made more precise about what went wrong:

• The loader now treats ENOTDIR the same as ENOENT as a "file not found" condition.
• The parsing result now always carries a present: true marker, including on the malformed- or oversized-file error path, and logs telemetry with a scope payload.
• On failure, the parser now discriminates between an invalid-schema failure (reported as mcp_config_invalid_schema) and other parse failures, replacing a telemetry call that had unconditionally reported success.

Why

This gives more accurate feedback about MCP config problems, so a missing file, a malformed file, and a file that fails schema validation can be told apart instead of being lumped into one generic failure.