Hover is broken in `monaco-graphql`. On the first line it throws and
renders nothing; on later lines it doesn't crash but highlights the
wrong token.

`GraphQLWorker.doHover` passes a 0-indexed position to `getRange`, which
actually wants a 1-indexed `SourceLocation` (per the GraphQL spec, and
what its other callers pass via `error.locations[0]`). On line 0 the
loop never runs and `Expected Parser stream to be available` fires. On
later lines `getRange` parses to end-of-line and ignores `column`, so
the range comes back pointing at the previous line's last token.

`getRange` was the wrong tool here anyway. It was built for diagnostics,
not for finding the token under a cursor. `getTokenAtPosition` is what
we want, and it's what the rest of the language service already uses.

Fixes #4157
Fixes #3168

Supersedes #4162, which tried to special-case `line === 0` inside
`getRange` itself. That left `getRange` returning a `Position` with
`line: -1` (which becomes `0` after `toMonacoRange`'s `+1`, invalid in
Monaco's 1-indexed API), and didn't address the bigger problem that
`getRange` doesn't compute a token range to begin with.

## Validation

```bash
yarn install
yarn build
yarn workspace example-monaco-graphql-react-vite dev
```

Open the URL Vite prints, with browser devtools open.

1. Hover the word `query` on the first line. Before: console logs
`Error: Expected Parser stream to be available` and no popup appears.
After: popup renders and `query` is highlighted.
2. Press Enter to push the query down a line, then hover a field name.
Before: popup appears but the highlight is on the previous line's last
token. After: the field under the cursor is highlighted.