test: add end-to-end Gitea compatibility suite (#1180)

Adds a real-Gitea compatibility suite against stable and nightly to catch runner and API drift before release.

The suite shares one Gitea and regular runner, using repository runners only for cache v1/v2 and ephemeral behavior. It covers registration, payload and log encoding, secrets, variables, services, artifacts, outputs, matrices, cache, dispatch, live logs, cancellation, and ephemeral teardown.

CI runs both images in parallel. Warm local timings:

| Image | Before | After |
| --- | ---: | ---: |
| `gitea/gitea:latest` | ~70s | 26.54s |
| `gitea/gitea:main-nightly` | ~54s | 22.53s |

The former serial matrix took about 2 minutes. Parallel suite execution is now bounded by the slower ~26.5-second variant.

Shared Renovate matcher: https://gitea.com/gitea/renovate-config/pulls/552

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1180
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: bircni <bircni@icloud.com>

e2e/testdata/workflows/dispatch.yml

@@ -0,0 +1,11 @@
+name: dispatch
+on:
+  workflow_dispatch:
+    inputs:
+      subject:
+        required: true
+jobs:
+  greet:
+    runs-on: ubuntu-latest
+    steps:
+      - run: test "${{ inputs.subject }}" = dispatch-input-value