Refactor: move CE project selection to job creation time

[ElePT]

Summary

This PR is built on top of the changes from #2108 and is currently blocked by it.

Up until now the gateway selected a Code Engine project the first time the scheduler touched a job, deep inside FleetsRunner. This PR moves that decision to RunJobSerializer.create(), the moment the job is written to the database.

The zone-lookup logic (reading FLEETS_PROFILE_ZONE_MAP and querying CodeEngineProject by zone) moves into a new select_ce_project() helper in core/model_managers/. The serializer calls it for every Fleets job and writes the chosen project onto the job record in the same save, so the runner never has to figure this out. If no suitable project exists the API returns a 400 right away instead of the job sitting in the queue and failing later.

On the runner side, _get_or_assign_project() is replaced by a much simpler _get_project() that just reads and validates whatever the serializer already put there.

Details and comments

This PR also fixes thesync_ce_project management command to define two separate initialization paths:

• a single_project path that uses the existing CE_PROJECT_ID variable. This path assumes automatic zone selection is available for resource provisioning (which is not the case currently, but should be in the future).
• a multi-project path with 2 new variables:CE_ZONE lets single-project deployments pin their project to an availability zone, and CE_PROJECTS accepts a JSON array for multi-zone setups where each zone has its own CE project.

[Tansito]

Initially the code here looks good to me but I would like to take a better look as soon as we merge #2108 . With less changes it will be quite easier.