feat: add variable interpolation support for WebSocket requests

[chirag-bruno]

Description

• Add WebSocket body interpolation in interpolateVars function
• Interpolate URL, headers, and all messages in request.body.ws array with full variable context
• Refactor sendWsRequest to use main process preparation (removes duplication)
• Add mode property to wsRequest object for proper request type detection
• Ensure consistent variable precedence matching HTTP/gRPC requests
• Centralise all interpolation logic in main process via prepareWsRequest

This PR addresses this JIRA ticket.

Contribution Checklist:

• [X] The pull request only addresses one issue or adds one feature.
• [X] The pull request does not introduce any breaking changes
• [ ] I have added screenshots or gifs to help explain the change if applicable.
• [X] I have read the contribution guidelines.
• [X] Create an issue and link to the pull request.

[sid-bruno]

Add tests in playwright for this as well