fix(cli): include path in mcp config parse errors

[OnlyYu1996]

Summary

• include the MCP config file path in TOML parse errors
• route MCP config reads through a shared parser helper so add/get/list/remove/auth paths report the source file consistently
• add regression coverage for parse errors including the config path

Fixes PlatformNetwork/bounty-challenge#52424

Validation

• cargo +stable fmt --package cortex-cli --check
• cargo +stable test --package cortex-cli mcp_cmd::config::tests::test_parse_config_content_error_includes_path -- --nocapture
• cargo +stable test --package cortex-cli mcp_cmd:: -- --nocapture (191 passed)
• cargo +stable check --package cortex-cli --lib
• cargo +stable build --package cortex-cli --bin Cortex
• git diff --check

[greptile-apps]

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