Route integration-tests trigger through emu-access runner (#769)

## Summary

Moves the `trigger-tests` job in
`.github/workflows/integration-tests.yml` from the
`databricks-deco-testing-runner-group` to
`databricks-release-runner-group-emu-access`, so the cross-org GitHub
App installation lookup against `databricks-eng` can succeed.

| | Before | After |
|---|---|---|
| Runner group | `databricks-deco-testing-runner-group` |
`databricks-release-runner-group-emu-access` |
| Label | `ubuntu-latest-deco` | `linux-ubuntu-latest-emu-access` |

## Why

Between **2026-04-17** and **2026-04-20**, the `databricks` org
tightened its IP allow list. Since then, every PR-triggered Integration
Tests run on this repo has failed:

- `create-github-app-token` resolves the installation for `${{
secrets.ORG_NAME }}` (`databricks-eng`) by calling
`/repos/databricks-eng/.../installation`.
- The deco runner group's egress IPs are no longer on the allow list, so
that call returns **403**.
- The downstream `gh workflow run sdk-java-isolated-pr.yml -R
databricks-eng/...` dispatch never happens.
- Merges still go through only because `merge_group` auto-approves the
check without running tests.

The `emu-access` runner pool's egress IPs **are** on the allow list, so
the installation lookup succeeds from there.

## Where the changes come from

Ported from the Go SDK fix:
[databricks/databricks-sdk-go#1638](databricks/databricks-sdk-go#1638)
(supersedes #1616, verified green end-to-end on 2026-04-10).

The Go PR splits `trigger-tests` into two jobs:

- **`create-check`** — stays on deco, creates a same-org check run via
the `databricks/...` check-runs API.
- **`trigger-tests`** — moves to emu-access, performs the cross-org
`workflow_dispatch`.

## What this PR does differently

The Java SDK's `integration-tests.yml` **does not create a check run**
in the PR flow — `trigger-tests` only performs the cross-org dispatch.
Nothing needs to stay on the deco runner, so the equivalent minimal fix
is a single runner-group change on the existing job rather than a
two-job split. No job splitting, added dependencies, or `check_run_id`
plumbing was needed here.

Matches the equivalent Python SDK fix:
[databricks/databricks-sdk-py#1396](databricks/databricks-sdk-py#1396).

## How is this tested

This PR's own Integration Tests run is the test. Expected outcome:

- [ ] `trigger-tests` runs on the emu-access runner and
`create-github-app-token` succeeds (no 403).
- [ ] A `sdk-java-isolated-pr` `workflow_dispatch` event appears on
`databricks-eng/eng-dev-ecosystem`.
- [ ] The Integration Tests check on this PR transitions to `success` /
`failure` based on the dispatched run.

---

`NO_CHANGELOG=true`

This pull request and its description were written by Isaac.

Author: mihaimitrea-db

.github/workflows/integration-tests.yml

@@ -33,8 +33,8 @@ jobs:
     name: Trigger Tests
 
     runs-on:
-      group: databricks-deco-testing-runner-group
-      labels: ubuntu-latest-deco
+      group: databricks-release-runner-group-emu-access
+      labels: linux-ubuntu-latest-emu-access
 
     needs: check-token
     if: github.event_name == 'pull_request'  && needs.check-token.outputs.has_token == 'true'