fix: validate run attach server url

[Bortlesboat]

Summary

• Validate cortex run --attach values before falling back to local execution.
• Reject filesystem-looking values such as /nonexistent/file.txt with a clear HTTP(S) server URL error.
• Keep valid HTTP server URLs working with the existing dry-run/fallback behavior.

Why

This addresses PlatformNetwork/bounty-challenge#53338. --attach is documented as a remote Cortex server URL, but the current implementation accepts local-looking paths and then ignores them when server attachment falls back to local mode. That can make users think a file was included when it was not.

Verification

• cargo test -p cortex-cli --test run_attach
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

[greptile-apps]

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