Add all_evidence to content service and cascade upload state to Evidence

[caitmich]

Summary

• Adds Dradis::Plugins::ContentService::Evidence#all_evidence, mirroring the existing Issues#all_issues and ContentBlocks#all_content_blocks:

def all_evidence
  case scope
  when :all
    project.evidence
  when :published
    project.evidence.published
  else
    raise 'Unsupported scope!'
  end
end

This lets export plugins fetch Evidence already scoped to the export's :published/:all scope, the same way they already do for Issues and Content Blocks, instead of re-querying node.evidence/issue.evidence themselves inside export processors.

• Also fixes create_evidence so it cascades the upload's chosen :state to newly created Evidence records, matching create_issue's existing behaviour

(Needed by the Word & Excel export PRs (dradis-pro#2938, dradis-pro#2951) and by html_export/csv_export (#99, #57), all of which fetch all_evidence once per export and group/filter it in memory per Issue rather than re-querying per Issue. Evidence's independent published/draft/ready_for_review review state was added in dradis-ce/dradis-pro branch qa/add-evidence-review-state; this PR requires Evidence#published (a Rails enum scope), which that branch adds, to exist on the host app's Evidence model.)

Testing Steps

From the Dradis Pro app directory, with this gem checked out at a sibling path, temporarily point the Gemfile at this branch/path and run:

bin/rspec [dradis-plugins path]/spec/lib/dradis/plugins/content_service/evidence_spec.rb

Assert all examples pass, confirming: all_evidence returns only published evidence by default and all evidence when scope is :all; create_evidence sets the new record's state from the :state argument (or the service's default state when none is given); and re-uploading matching content does not change an existing evidence record's state.

Other Information

Once merged, this needs a version bump + gem release, and the dependent PRs above need their temporary local path/branch overrides in the Gemfile switched to the released version.

I assign all rights, including copyright, to any future Dradis
work by myself to Security Roots.

Check List

• Added a CHANGELOG entry
• Added specs