Request textDocument/hover fails when top-level env key is incomplete

Type: Bug

Making a workflow file like this:

```yaml
name: Sample 1
on:
- push
env:
jobs:
test:
runs-on: ubuntu-latest
steps:
- run: echo "Hello, ${{ env.TEST }}"
```

Causes the following error when hovering over the `- run:` line:

```
[Error - 4:08:42 PM] Request textDocument/hover failed.
Message: Request textDocument/hover failed with message: Unexpected type 'NullToken' encountered while reading 'workflow env'. The type 'MappingToken' was expected.
Code: -32603
```

In contrast, **none** of these workflows cause the error:

```yaml
name: Sample 1
on:
- push
jobs:
test:
runs-on: ubuntu-latest
env:
steps:
- run: echo "Hello, ${{ env.TEST }}"
```

```yaml
name: Sample 1
on:
- push
jobs:
test:
runs-on: ubuntu-latest
steps:
- run: echo "Hello, ${{ env.TEST }}"
env:
```

Extension version: 0.27.1
VS Code version: Code 1.100.2 (848b80aeb52026648a8ff9f7c45a9b0a80641e2e, 2025-05-14T21:47:40.416Z)
OS version: Darwin arm64 24.4.0
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|Apple M4 Max (14 x 2400)|
|GPU Status|2d_canvas: enabled
canvas_oop_rasterization: enabled_on
direct_rendering_display_compositor: disabled_off_ok
gpu_compositing: enabled
multiple_raster_threads: enabled_on
opengl: enabled_on
rasterization: enabled
raw_draw: disabled_off_ok
skia_graphite: disabled_off
video_decode: enabled
video_encode: enabled
webgl: enabled
webgl2: enabled
webgpu: enabled
webnn: disabled_off|
|Load (avg)|3, 4, 4|
|Memory (System)|36.00GB (2.98GB free)|
|Process Argv|--crash-reporter-id 1eb59a8f-a777-446b-b97e-fe7c3d092446|
|Screen Reader|no|
|VM|0%|

A/B Experiments

```
vsliv368cf:30146710
vspor879:30202332
vspor708:30202333
vspor363:30204092
pythonvspyt551cf:31249601
vscod805cf:30301675
binariesv615:30325510
c4g48928:30535728
azure-dev_surveyone:30548225
a9j8j154:30646983
962ge761:30959799
h48ei257:31000450
pythontbext0:30879054
cppperfnew:31000557
dwnewjupyter:31046869
pythonrstrctxt:31112756
nativeloc2:31192216
5fd0e150:31155592
dwcopilot:31170013
6074i472:31201624
dwoutputs:31242946
customenabled:31248079
hdaa2157:31222309
copilot_t_ci:31222730
e5gg6876:31282496
pythoneinst12:31285622
bgtreat:31268568
4gafe986:31271826
c7cif404:31309971
pythonpulldiagcf:31298646
996jf627:31283433
pythonrdcb7:31303018
usemplatestapi:31297334
0aa6g176:31307128
7bj51361:31289155
747dc170:31275177
pylancecolorcf:31309109
aj953862:31281341
generatesymbolt:31295002
convertfstringf:31295003
gendocf:31295004

```

Start by reproducing the textDocument/hover request in VS Code with the workflow examples, focusing on the top-level env case and its NullToken error. Trace the hover handling in the extension; done means hovering over the run step no longer fails for an incomplete top-level env key, while the other examples continue to work.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.