fix(cli): trim trailing slash from proxy server

[EfeDurmaz16]

Summary

• normalize proxy server values by trimming trailing slashes before passing them to Chrome
• keep proxy auth parsing intact for authenticated proxy URLs
• add focused parser coverage for plain, authenticated, and no-scheme proxy values

Fixes #1349.

Verification

• cargo test --manifest-path cli/Cargo.toml test_parse_proxy
• cargo fmt --manifest-path cli/Cargo.toml -- --check
• cargo clippy --manifest-path cli/Cargo.toml --all-targets -- -D warnings
• git diff --check

[vercel]

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