Reject non-GET WebSocket methods

[ryanfowler]

Summary

• Reject explicit non-GET --method values during WebSocket validation instead of warning and continuing
• Remove the obsolete WebSocket method warning path in runtime execution
• Update WebSocket unit and integration coverage to assert the new error behavior

Testing

• Unit tests added for WebSocket exclusives validation
• Integration tests updated and verified for the WebSocket dry-run and noninteractive cases
• cargo fmt, cargo clippy --locked --all-targets --all-features -- -D warnings, cargo test --all-features, and the serialized integration suite all passed