Fix wrong behavior when CI keyword "when" is an array

What does this MR do and why?

Related to #347356 (closed)

We have the "inclusion" validation for the when keyword but no validation for its type. So, the attribute also accepts an array as the value. The "inclusion" validation of ActiveModel::Validations also accepts array values and compares each element.

class MyTestClass
  include ActiveModel::Validations

  attr_accessor :title

  validates_inclusion_of :title, in: %w[abc def]
end

t = MyTestClass.new
t.title = 'abc'
t.valid? # => true

t.title = 'ghi'
t.valid? # => false

t.title = ['abc']
t.valid? # => true

In this MR, we are adding the type: String validation for the when keyword for Entry::Job and Entry::Bridge entries.

Notes:

• In the issue, we discussed if this change can be a breaking change for some users but this behavior is already broken so it should be okay to fix this without considering breaking changes.
• This MR does not aim to fix the duplication between Entry::Job and Entry::Bridge. Maybe in another issue or maybe we should leave them as they are to keep them clear.
• Maybe we should have a model validation for Ci::Processable#when but it requires additional work.
• We have other when attributes for other CI entries (Entry::Change and Entry::Artifacts). Maybe we should also fix them in a follow-up. (#378368 (closed))
• We may need to check every inclusion validation in every CI entry. (#378368 (closed))

Screenshots or screen recordings

build:
  when: [always]
  script: exit 0

Before:

The job is skipped because in Ci::ProcessBuildService, the job falls into the "else" case.

After:

Correct message 💯

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