s3vectors: name weed-vector processes servers (#12627)

* s3vectors: call the registered weed-vector processes servers

The gateway's discovery named them workers, the name of the admin
plugin workers. Also drops two comments left without a function.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* test/s3vectors: build the Enterprise engine as weed-vector

In an Enterprise checkout, make ./weed-vector builds the distributed
engine, which also serves VectorService, instead of a separate
weed-vector-worker. An open-source checkout builds weed-vector-server
as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* test/s3vectors: distributed mode starts two more weed-vector engines

It started weed-vector-worker, which no target builds now, and waited
for a gateway log line that no longer exists. The Enterprise
weed-vector's engines all join the placement controller.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -147,7 +147,6 @@ telemetry/server/telemetry-server
 __pycache__/
 /test/s3vectors/weed
 /test/s3vectors/weed-vector
-/test/s3vectors/weed-vector-worker
 /test/s3vectors/weed-vector-cli
 /test/s3vectors/weed-worker
 /test/s3vectors/weed-volume