Fix JWT token expiration when memoized before timeout is set

What does this MR do and why?

The JWT job token (encoded_jwt) is memoized on the build object. When pre_assign_runner_checks runs before the job transitions to running, it can trigger token generation via:

• has_valid_build_dependencies? -> scoped_variables -> kubernetes_variables_from_job
• secrets_provider_not_found? (EE) -> scoped_variables -> kubernetes_variables_from_job

The kubernetes_variables_from_job method calls job.try(:token), which memoizes the JWT with the current timeout_value. However, at this point update_timeout_state hasn't run yet (it runs during pending->running transition), so timeout_value may be nil, resulting in a JWT with only ~6 minutes TTL (60s default + 5min leeway) instead of the job's actual timeout.

This fix clears the encoded_jwt memoization in present_build! (which runs after the job has transitioned to running and timeout is set), ensuring the token is regenerated with the correct expiration.

This particularly affects jobs with needs: dependencies, as they are more likely to trigger the dependency validation code path.

References

#581924 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Minimal reproducer:

# .gitlab-ci.yml
setup:
  script: echo "Setup done"

trigger-child:
  trigger:
    include: child.yml
    strategy: depend

and child pipeline:

# child.yml
child-job:
  image: ruby
  needs:
    - pipeline: $CI_UPSTREAM_PIPELINE_ID
      job: setup
  script: ruby -e "require 'base64'; puts Base64.encode64(ENV['CI_JOB_TOKEN'])"
  timeout: 1h

Then copy the encoded string into a variable s in a gdk rails console and run:

jwt = ::Ci::JobToken::Jwt.decode(Base64.decode64(s)).instance_variable_get(:@jwt)
puts (Time.at(jwt.payload["exp"]) - Time.now) / 60

Expected results:

• On master: You get under 6 minutes
• On this branch: You get on the order of 60 minutes (depending on how quick you are)

This should be over 60 because we have a 1h timeout and then some leeway. But on master, I get just 6 minutes!

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