[BUG] VS Code extension login screen: "Bedrock, Foundry, or Vertex" link uses a non-existent docs anchor (#using-third-party-providers)

## Summary

On the VS Code extension's sign-in screen, the third button, **Bedrock, Foundry, or Vertex**, opens

```
https://code.claude.com/docs/en/vs-code#using-third-party-providers
```

That anchor does not exist on the page. The heading is `## Use third-party providers`, so the anchor Mintlify generates is `#use-third-party-providers`. The page's own prose links to it correctly (in *Prerequisites*: "see [Use third-party providers](#use-third-party-providers) for setup instructions"), so only the button in the extension is out of date.

Effect: the link still loads the page, but it lands at the top instead of the setup section, so a user who clicks a button labelled "Instructions on how to use API keys on third party providers" gets no instructions in view and has to scroll the whole page to find them.

## Expected

The button links to `https://code.claude.com/docs/en/vs-code#use-third-party-providers` and the browser jumps to the "Use third-party providers" section.

## Actual

The button links to `#using-third-party-providers`, an anchor with no matching heading, so the browser ignores the fragment and shows the top of the page.

## Steps to reproduce

1. Open the Claude Code extension in VS Code while signed out, so the "How do you want to log in?" screen is shown.
2. Click **Bedrock, Foundry, or Vertex** (the third button, with the external-link icon).
3. Look at the URL in the browser: it ends with `#using-third-party-providers`, and the page is scrolled to the top.

## Environment

- Claude Code 2.1.246
- VS Code extension, Windows 11 Pro (10.0.26200)

## Suggested fix

Change the URL behind that button from `#using-third-party-providers` to `#use-third-party-providers`. Alternatively, add a redirect/alias anchor on the docs page if the old fragment is linked from elsewhere too.

Search the VS Code extension's signed-out sign-in screen implementation for the "Bedrock, Foundry, or Vertex" button and inspect its documentation URL. Change the fragment to #use-third-party-providers, then reproduce the flow and confirm the browser opens the "Use third-party providers" section.

Written by the indexing model from the issue text.