ci(audience): add selfhosted_only flag for workflow_dispatch (SDK-255)

- Adds a workflow_dispatch input selfhosted_only (boolean, default false)
  that, when true, skips the playmode (Win/macOS), playmode-linux (GameCI),
  and mobile-build jobs so only the self-hosted Linux PlayMode cells run.
- Resolves the Unity seat-pool contention that has been failing the manual
  self-hosted Linux validation. With all 22 jobs firing simultaneously the
  seat pool fills before the self-hosted Linux job can activate; with the
  flag set only the 2 self-hosted Linux cells need a seat.
- Default-false preserves the existing "workflow_dispatch fires everything"
  behaviour for casual manual triggers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

.github/workflows/test-audience-sample-app.yml

@@ -13,14 +13,19 @@ on:
       - 'examples/audience/ProjectSettings/**'
       - '.github/workflows/test-audience-sample-app.yml'
   workflow_dispatch:
+    inputs:
+      selfhosted_only:
+        description: Run only the self-hosted Linux PlayMode validation (skip Win, macOS, GameCI Linux, mobile)
+        type: boolean
+        default: false
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
 jobs:
   playmode:
-    if: github.event.pull_request.head.repo.fork == false || github.event_name == 'workflow_dispatch'
+    if: github.event.pull_request.head.repo.fork == false || (github.event_name == 'workflow_dispatch' && inputs.selfhosted_only != true)
     name: ${{ matrix.target }} / ${{ matrix.backend }} / Unity ${{ matrix.unity }}
     strategy:
       fail-fast: false
@@ -440,7 +445,7 @@ jobs:
   # Linux PlayMode runs on GitHub-hosted Ubuntu via GameCI Docker, not on the
   # self-hosted matrix above, to keep self-hosted machines free for Win/macOS.
   playmode-linux:
-    if: github.event.pull_request.head.repo.fork == false || github.event_name == 'workflow_dispatch'
+    if: github.event.pull_request.head.repo.fork == false || (github.event_name == 'workflow_dispatch' && inputs.selfhosted_only != true)
     name: ${{ matrix.target }} / ${{ matrix.backend }} / Unity ${{ matrix.unity }}
     runs-on: ubuntu-latest-8-cores
     strategy:
@@ -504,7 +509,7 @@ jobs:
   # Note: Unity 6 cells use 6000.4.5f1 (the latest patch with a GameCI image);
   # 6000.4.0f1 does not have a published GameCI Docker image.
   mobile-build:
-    if: github.event.pull_request.head.repo.fork == false || github.event_name == 'workflow_dispatch'
+    if: github.event.pull_request.head.repo.fork == false || (github.event_name == 'workflow_dispatch' && inputs.selfhosted_only != true)
     name: ${{ matrix.target }} / IL2CPP / Unity ${{ matrix.unity }}
     runs-on: ubuntu-latest-8-cores
     strategy: