Fix plugin list JSON error for conflicting filters

[OnlyYu1996]

Summary

• emit a structured JSON error when cortex plugin list --json --enabled --disabled is used
• keep the non-JSON conflicting filter path unchanged
• fix a stale test import so cortex-cli tests compile on current main

Fixes PlatformNetwork/bounty-challenge#52983

Verification

• cargo fmt --package cortex-cli --check
• cargo test --package cortex-cli plugin_cmd::tests::test_plugin_list_filter_conflict_error_serializes_as_json -- --nocapture
• cargo test --package cortex-cli plugin_cmd::tests -- --nocapture
• cargo check --package cortex-cli --lib
• git diff --check
• cargo run --package cortex-cli --bin Cortex -- plugin list --json --enabled --disabled

[greptile-apps]

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