fix(plugin): align empty name errors

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53115.

Changes:

• Uses one shared empty plugin-name validation path for plugin install and plugin new.
• Keeps the error text consistent: Plugin name cannot be empty.
• Treats whitespace-only plugin new names as empty, matching install behavior.
• Adds a regression test covering both command paths.
• Fixes a stale test import so the cortex-cli test target builds on current main.

Validation:

• cargo +stable fmt --package cortex-cli --check
• cargo +stable test --package cortex-cli plugin_cmd::tests::test_plugin_install_and_new_empty_name_errors_match -- --nocapture
• cargo +stable check --package cortex-cli --lib
• git diff --check

[greptile-apps]

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