fix: include snapshot diff in json output

[Bortlesboat]

Summary

• Include diff metadata in cortex debug snapshot --json --diff output instead of dropping the flag on the JSON path.
• Keep the existing placeholder behavior explicit by returning a not_implemented diff status and message.
• Add CLI coverage that exercises debug snapshot --json --diff and asserts the JSON diff metadata is present.

Related to PlatformNetwork/bounty-challenge#45699.

Verification

• cargo test -p cortex-cli --test debug_snapshot_json_diff debug_snapshot_json_includes_diff_when_requested -- --nocapture
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

[greptile-apps]

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