Synced from cloud PR: Skyvern-AI/skyvern-cloud#9042
Author: @wintonzheng

## Summary

- Enables file download capture via CDP Fetch domain for remote browser connections (`browser_address`)
- Downloads are streamed over WebSocket to the Skyvern worker disk, eliminating the need for a separate file server
- Adds `browser serve` CLI command for easy local Chrome + ngrok setup

## Changes

1. **cloud/webeye/special_browsers.py**: Add CDPDownloadInterceptor to `create_persistent_cdp_browser` - this is the function called for remote browser connections with `browser_address`

2. **skyvern/cli/commands/browser.py**: Add `browser serve` command that launches:
   - Local Chrome with CDP enabled
   - Unified server that proxies CDP WebSocket and optionally serves files

3. **skyvern/forge/sdk/artifact/storage/s3.py & azure.py**: Update `save_downloaded_files` to use local directory. The `browser_address` and `browser_api_key` parameters are kept for backward compatibility but are no longer used since CDPDownloadInterceptor handles downloads.

4. **skyvern/forge/agent.py**: Add explanatory comments about CDPDownloadInterceptor handling remote browser downloads

5. **skyvern/webeye/browser_factory.py**: Add CDPDownloadInterceptor support with `enable_cdp_download_interceptor` parameter

## Test plan

- [x] End-to-end test with local Skyvern + browser serve + ngrok
- [x] Verified CDPDownloadInterceptor is enabled on remote browser pages
- [x] Workflow completes successfully with remote browser connection
- [x] Compile checks pass
- [x] Pre-commit hooks pass

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