fix(cli): align mcp existing server advice

[OnlyYu1996]

Fixes inconsistent advice when cortex mcp rename targets an existing MCP server.

Bounty issue: PlatformNetwork/bounty-challenge#53102

Changes:

• Share the existing-server conflict advice between mcp add and mcp rename.
• Add mcp rename --force / -f so the advice points to a working overwrite path.
• Cover clap command validity and rename force parsing with tests.
• Keep the current main test import fix needed for cortex-cli tests/checks to pass.

Validation:

• cargo +stable fmt --package cortex-cli --check
• cargo +stable test --package cortex-cli mcp_cmd::types::tests::test_mcp_cli_command_aliases_are_valid -- --nocapture
• cargo +stable test --package cortex-cli mcp_cmd:: -- --nocapture
• cargo +stable check --package cortex-cli --lib
• cargo +stable build --package cortex-cli --bin Cortex
• real CLI smoke for add conflict, rename conflict, and rename --force
• git diff --check

[greptile-apps]

Your free trial has ended. If you'd like to continue receiving code reviews, you can add a payment method here.