fix(ci): add owpenbot sidecar stub to build.rs and enable Bun on all platforms

- Add ensure_owpenbot_sidecar() to build.rs to create debug stubs when binary missing
- Remove 'if: matrix.os_type == windows' from Bun setup in CI workflows
- This fixes PreRelease builds failing because prepare:sidecar needs bun to compile owpenbot

.github/workflows/prerelease.yml

@@ -121,7 +121,6 @@ jobs:
           version: 10.27.0
 
       - name: Setup Bun
-        if: matrix.os_type == 'windows'
         uses: oven-sh/setup-bun@v1
         with:
           bun-version: "1.1.29"

.github/workflows/release-macos-aarch64.yml

@@ -219,7 +219,6 @@ jobs:
           version: 10.27.0
 
       - name: Setup Bun
-        if: matrix.os_type == 'windows'
         uses: oven-sh/setup-bun@v1
         with:
           bun-version: "1.1.29"