feat: sub agent use new supervisor [NR-497494]

[sigilioso]

Summary

This PR introduces a refactored version of sub_agent.rs in a new temporary module sub_agent_new.rs. The refactoring adopts the new Supervisor trait that unifies SupervisorStopper functionality and adds support delegating to an existing supervisor the application of new configuration via the apply() method.

Changes

Supervisor apply() usage

Previous behavior:

• When a new remote config arrived, the old supervisor was always stopped and a new one was always started
• This means long interruptions of the agents while installing new agent on 'on-host' environments.

New behavior:

• When a new remote config arrives and a supervisor is already running, the system attempts to apply the configuration without restarting using Supervisor::apply()
  • If there's no existing supervisor does the system fall back to the old "stop and start" approach
  • If apply() fails, the RemoteConfig is reported as failed and no supervisor keeps running (agents stop).
• This enables zero-downtime updates if implemented in the supervisor.

Supervisor Lifecycle

Previous behavior:

// Old: Explicit stop before starting new supervisor
stop_supervisor(old_supervisor);
new_supervisor.start(...)

New behavior:

// New: Try to apply first, only rebuild if necessary
if let Some(existing) = old_supervisor {
    existing.apply(new_config) // Hot-reload attempt
} else {
    build_and_start_new_supervisor()
}

‼️ The Supervisor::apply() method is responsible for handling any necessary internal restarts.

Small refactors and improvements

• Simplify error handling: SupervisorCreationError is kept on init_supervisor only.
• Add new helpers for improving maintainability
• Unit tests are extended to check the configuration updates behaviors when there was a previous supervisor ("aply") and there was no supervisor ("build + start").
• Extend helpers documentation.

Notes for reviewers

• Because the new behavior is introduced in a different module, this PR is best reviewed commit-by-commit.

• The new module is not yet used and will replace sub_agent.rs once other dependent changes are ready.

  • Adapting the k8s supervisor feat: impl new supervisor traits k8s #2055
  • Adapting the on-hots supervisor

  When the supervisors are ready we need to:

  1. Replace sub_agent.rs with sub_agent_new.rs
  2. Fix the imports
  3. Replace the supervisors as well

[DavSanchez]

Needs conflict resolution (probably because we both added mocks to the supervisor module xD)

[sigilioso]

That was it! Thanks for pointing out

[sigilioso]

@DavSanchez I opted for the incoming implementation for the mocks because sub_agent_new.rs is already using them. They were mostly the same! But I can switch names or whatever if you think we should to 🙂

[DavSanchez]

Fine by me! 👌

[gsanchezgavier]

I ended up doing a diff between sub_agent and sub_agent_new files locally 😅 .

Haven't check the tests yet but changes look good . I left a doubt

[DavSanchez]

Looking good! Some nits now that I'm getting to look at this code again. Feel free to not address any at all xD

Let me have another shot at the tests before approving.

[DavSanchez]

💯

[gsanchezgavier]

great! i did a diff between the new and old sub-agents and foucus the review on trying to catch any behave issue but it looks all good to me! , i saw we could reuse most of the testing scenarios with some adaptation :) 🚀