[FE] Treat an archived tag as no longer part of its taxonomy on create, import, and export

User Story

As a platform administrator, I want a tag value that belongs to an archived tag to explain why it’s unavailable rather than failing with a generic duplicate error, in order to understand what happened and pick a value I can actually use.

Acceptance Criteria

This ticket covers three surfaces, not one: adding a tag interactively through the Taxonomy Editing UI (and the API/model method it calls), importing a file, and exporting a taxonomy.

Adding a tag via the Taxonomy Editing UI

Scenario: Adding a tag whose value belongs to an archived tag explains what happened
  Given a taxonomy with an archived tag whose value is "Photosynthesis"
  When an administrator adds a tag with the value "Photosynthesis"
  Then the tag is not created
  And the error says the value belongs to an archived tag and that a different value should be used

Scenario: Adding a tag whose value is genuinely taken still reports a duplicate
  Given a taxonomy with an unarchived tag whose value is "Photosynthesis"
  When an administrator adds a tag with the value "Photosynthesis"
  Then the tag is not created
  And the error reports the duplicate as it does today

Scenario: Adding a tag whose value is free succeeds
  Given a taxonomy with no tag of any kind holding the value "Respiration"
  When an administrator adds a tag with the value "Respiration"
  Then the tag is created

Importing a file

Scenario: Importing a value that belongs to an archived tag is refused
  Given a taxonomy with an archived tag whose value is "Photosynthesis"
  When a file containing that value is imported into that taxonomy
  Then the archived tag is not updated or revived by the import
    And the import reports that the value belongs to an archived tag and a different value should be used

Scenario: Importing an external identifier that belongs to an archived tag is refused
  Given a taxonomy with an archived tag holding a given external identifier
  When a file containing that external identifier is imported into that taxonomy
  Then the archived tag is not updated or revived by the import
    And the import reports that the identifier belongs to an archived tag

Scenario: An import that collides changes nothing else
  Given an import file containing one colliding value and several valid new tags
  When the file is imported
  Then the collision is reported
    And the taxonomy is left as it was before the import

Exporting a taxonomy

Scenario: An archived tag is absent from an export
  Given a taxonomy with one archived tag and two that are not archived
  When the taxonomy is exported
  Then only the two unarchived tags appear in the exported output

Scenario: Re-importing an export of a taxonomy containing archived tags is clean
  Given a taxonomy containing archived tags
  When it is exported and the result imported into an empty taxonomy
  Then the import succeeds
    And the resulting taxonomy contains only the tags that were not archived

Description

Once tags can be archived, a taxonomy contains rows that are invisible on every read path but still occupy their value and their external identifier. Creating a tag with one of those values fails today with the generic message for a value that already exists, which is confusing: the administrator can see no such tag anywhere and has no way to work out why the value is refused.

Import has the same problem and a worse failure mode. Its usual behavior on encountering an existing value or external identifier is to update the matching row, so without a change it would silently write to an archived tag and effectively revive it as a side effect of an import.

The decision is that an archived tag is treated as no longer part of the taxonomy, on every surface that can encounter one: the Taxonomy Editing UI, import, and export. Export omits it entirely, and any attempt to create or import over its value or external identifier is refused with a message explaining why and telling the administrator to use a different value.

Start with src/openedx_tagging/api.py and add_tag_to_taxonomy, then trace the import plan and row matching under src/openedx_tagging/import_export/. Read the existing API, import, and export tests under tests/openedx_tagging/ before adding coverage for archived value and external-identifier collisions, atomic imports, and export round-tripping. Done means archived rows are never revived, collisions are reported as import errors, and exports contain only unarchived tags.