`Failed to fetch action metadata for actions/c/undefined: 'Not Found'`

Type: Bug

Create a workflow file and type:

```
jobs:
something:
runs-on: ubuntu-latest
steps:
- uses: actions/c
```

get completion for the end of `actions/c`

expected results: do not try to use `/undefined` -- I didn't type that and it's clearly a bug.

Extension version: 0.27.1
VS Code version: Code 1.97.2 (Universal) (e54c774e0add60467559eb0d1e229c6452cf8447, 2025-02-12T23:20:35.343Z)
OS version: Darwin arm64 24.3.0
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|Apple M1 Max (10 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)|8, 7, 7|
|Memory (System)|64.00GB (0.07GB free)|
|Process Argv|--crash-reporter-id 1fc67ee2-0174-4598-9f98-4537df0dd32c|
|Screen Reader|no|
|VM|0%|

A/B Experiments

```
vsliv368:30146709
vspor879:30202332
vspor708:30202333
vspor363:30204092
vswsl492:30256859
vscod805cf:30301675
binariesv615:30325510
py29gd2263:31024239
c4g48928:30535728
azure-dev_surveyone:30548225
vscrp:30673768
962ge761:30959799
h48ei257:31000450
pythontbext0:30879054
cppperfnew:31000557
dwnewjupytercf:31046870
nativerepl2:31139839
pythonrstrctxt:31112756
nativeloc2:31192216
iacca1:31171482
5fd0e150:31155592
dwcopilot:31170013
6074i472:31201624
dwoutputs:31242946
customenabled:31248079
5a9hg449:31230677
hdaa2157:31222309
copilot_t_ci:31222730
jda6j935:31233686

```

未指定原始檔案或測試。先使用 `actions/c` 在 VS Code 工作流程中重現完成請求，接著追蹤建構 `/undefined` 請求的 action-metadata 完成路徑。完成的標準是：不完整的參照不再產生包含 `/undefined` 的請求，且完成會在沒有所回報的 fetch 錯誤下返回。

由索引模型根據 Issue 內容生成。