[unified-server] Unified server tests break global CI run

[timswanson-google]

A PR that added the board server build into the dependency graph of broke CI runs because it caused the global test target to fail.

Original PR - #4145

Rollback PR - #4152

The problem seems to relate to the index.js file generated as part of the board server build being clobbered, since the build step is getting rerun concurrently.

[timswanson-google]

This issue can be seen in PR #4250, which is a minimal demonstration of the issue:

https://github.com/breadboard-ai/breadboard/actions/runs/13019338901/job/36316172993?pr=4250

[timswanson-google]

@aomarks has submitted a change to wireit to improve the error messaging in cases like this:

google/wireit#1270

[timswanson-google]

With the improved error messaging, the failed run now looks like this:

https://github.com/breadboard-ai/breadboard/actions/runs/13018501422/job/36313463374

❌ [packages/board-server:build:esbuild] Output file "/home/runner/work/breadboard/breadboard/packages/board-server/dist/server/index.js" was deleted unexpectedly. Is another process writing to the same location?

[timswanson-google]

For now, this issue has been worked around by removing the unified server test from the global test run. We should add it back once we root cause this issue and figure out how to address it.