fix(cli): make export --pretty meaningful

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53204.

Summary

• make cortex export JSON compact by default so --pretty has a visible effect
• narrow the help text to JSON and reject --pretty with YAML/CSV before reading session data
• keep YAML/CSV serialization unchanged when --pretty is not provided
• fix a stale test import that prevented the cortex-cli test binary from compiling

Validation

• cargo +stable check --package cortex-cli --lib
• cargo +stable fmt --package cortex-cli --check
• cargo +stable test --package cortex-cli export_cmd::tests (8 passed)
• git diff --check

[greptile-apps]

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