fix(core/xref): revert getUnversionedFallbacks, fix dedupeSpecContext empty-level bug

[Copilot]

getUnversionedFallbacks (introduced in #5226) was causing 12 CI karma test failures by injecting both versioned and unversioned spec shortnames as separate priority levels into xref API queries — e.g. [["service-workers-1"], ["service-workers"]]. The xref server doesn't implement true fallback-chain semantics; it searches all levels simultaneously and returns one result per matching shortname, so dual forms produced two results → "ambiguous dfn" → empty href.

A second bug in dedupeSpecContext unconditionally pushed even empty uniqueSpecs arrays (e.g. [["service-workers-1"], []]), which the server may interpret as "search all specs", compounding ambiguity.

Changes

• src/core/xref.js

  • Removed stripVersionSuffix and getUnversionedFallbacks
  • Reverted getSpecContext to push only the spec names as authored — no unversioned fallback injection
  • Fixed dedupeSpecContext to skip empty levels; switched back to Set-based O(1) dedup with an explicit if (uniqueSpecs.length) guard before pushing

• tests/spec/core/xref-spec.js

  • Consolidated the two service-workers tests added in fix(core/xref): normalize versioned spec names in data-cite context #5226 into a single "resolves terms with versioned data-cite on container" regression test for issue Same spec version resolution mess #5224 — the versioned shortname still resolves correctly without the fallback machinery