Remove spurious attributes from IsoSts models

[HassanAkbar]

Removes the 7 spurious attributes on 5 models that #42 deferred.

Five IsoSts models carried attributes ISOSTS does not define. They came from the earlier pass that verified attributes against the NISO XSD and applied the result to IsoSts — the wrong schema for these models.

Removed: type on graphic, content-type on body and back, and lang+specific-use on language and content-language.

Every removal is verified against ISOSTS.dtd and ISOSTS.xsd. %body-atts; expands to specific-use alone, back has no ATTLIST at all, %graphic-atts; gives xlink:type but never a bare type, and neither language nor content-language declares any attribute.

graphic keeps xlink:type. Only the unprefixed type was undeclared.

@id stays on all five. ISOSTS does not define it either, but that is the deliberate 86948b9 convention across 220 models.

Breaking change: 7 readers are gone — Body#content_type, Back#content_type, Graphic#graphic_type, and #lang/#specific_use on both Language and ContentLanguage. Callers reading them will get a NoMethodError. No fixture or reference document in the repo uses any of them, so on valid ISOSTS input these readers were always nil.

bundle exec rspec: 5212 examples, 0 failures. bundle exec rubocop: clean. All 7 new guards fail without the fix.

Metanorma PR checklist

• Breaking changes (list related PRs)
• Documentation update required (create task for this)
• External dependency introduced (documentation update need)
• Gem with native library introduced

refs #40

[Copilot]

Pull request overview

This PR aligns several Sts::IsoSts element models with the actual ISOSTS schema by removing attributes that were mistakenly carried over from NISO STS modeling, and strengthens the spec suite to ensure both the Ruby-level attribute sets and XML-level behavior reflect the schema.

Changes:

• Removed undeclared attributes from IsoSts models: Graphic#graphic_type, Body#content_type, Back#content_type, and #lang/#specific_use on Language and ContentLanguage.
• Updated attribute-key-set assertions to match the corrected models.
• Added XML-behavior guards (via Nokogiri) to ensure these undeclared XML attributes are ignored/dropped during parse/serialize.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
spec/iso_sts/iso_sts_element_spec.rb	Updates expected attribute sets and adds XML-level guards ensuring undeclared attributes are not preserved.
lib/sts/iso_sts/language.rb	Removes lang and specific_use attributes/mappings from Language.
lib/sts/iso_sts/content_language.rb	Removes lang and specific_use attributes/mappings from ContentLanguage.
lib/sts/iso_sts/graphic.rb	Removes the undeclared unprefixed type mapping (graphic_type) while keeping xlink:type.
lib/sts/iso_sts/body.rb	Removes the undeclared content-type mapping from Body.
lib/sts/iso_sts/back.rb	Removes the undeclared content-type mapping from Back.

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