Adds a read-only Status RPC to the gateway that returns the current
customer-friendly status of a previously submitted request, reconciled
from the append-only request log via
core/request.GetCurrentStateFromRequestLog. Status is a free-form string
(statuses can be added without breaking clients); unknown sqids map to a
typed RequestNotFoundError (user error) and empty sqids to ErrInvalidRequest.

Named "Status" to match the gateway's short RPC naming (Ping, Land, Status).

Changes:
- gateway/proto/gateway.proto: Status RPC, StatusRequest/StatusResponse, and
  RequestNotFoundError messages (regenerated protopb)
- gateway/controller/status.go: StatusController + tests
- example/server/gateway/main.go: wire controller (reuses existing RequestLogStore)

Validation:
- Unit tests: bazel test //gateway/controller:controller_test  (PASS)
- Integration (local stack): make local-gateway-start
  PORT=$(docker port submitqueue-gateway-service-1 8080 | head -1 | cut -d: -f2)

  # method is registered
  grpcurl -plaintext localhost:$PORT list uber.submitqueue.gateway.SubmitQueueGateway
    -> Land, Ping, Status

  # seed a request
  grpcurl -plaintext -d '{"queue":"test-queue","change":{"uris":["github:///pull/123/c3a4d5e6f7890123456789abcdef0123456789ab"]}}' \
    localhost:$PORT uber.submitqueue.gateway.SubmitQueueGateway/Land
    -> { "sqid": "test-queue/1" }

  # happy path
  grpcurl -plaintext -d '{"sqid":"test-queue/1"}' \
    localhost:$PORT uber.submitqueue.gateway.SubmitQueueGateway/Status
    -> { "status": "accepted" }

  # unknown sqid
  grpcurl -plaintext -d '{"sqid":"test-queue/999"}' \
    localhost:$PORT uber.submitqueue.gateway.SubmitQueueGateway/Status
    -> ERROR: request not found for sqid "test-queue/999"

  # empty sqid
  grpcurl -plaintext -d '{"sqid":""}' \
    localhost:$PORT uber.submitqueue.gateway.SubmitQueueGateway/Status
    -> ERROR: StatusController requires the request to have a sqid specified: invalid request

Co-authored-by: Cursor <cursoragent@cursor.com>