refactor: Collapse the four stylesheet endpoints into one

[rickbutterfield]

Summary

Stacked on #329 — this PR is rebased onto v5/refactor/preview-pipeline-dedup so it only shows its own diff. Once #329 merges to v5/dev, GitHub will auto-retarget this PR to v5/dev.

Collapses the four nearly-identical Get{Grid,List,Rte,SingleBlock}Stylesheets controller actions (and their four frontend counterparts) onto one parameterized implementation.

• New GET preview/stylesheets?blockType= action holds the real logic; the four existing plural routes become one-line forwarders to it. All four legacy routes are kept — this is additive only, not a breaking change, and IBlockPreviewService.GetStylesheetPaths's signature (a documented, overridable extension point) is completely untouched.
• The OpenAPI-generated TypeScript client was regenerated to pick up the new endpoint; all four preview elements now call it directly.
• blockType is validated ([BindRequired] + an explicit Enum.IsDefined check) so a missing or out-of-range value returns 400 instead of silently defaulting to Block Grid — a gap caught in whole-branch review, since this is a new public route on a NuGet-distributed package.
• Delegation tests verify not just that the right BlockType reaches the shared action, but that nodeKey/documentTypeUnique actually reach content resolution too.

Test plan

• dotnet test — 96/96 passing (backend branch's 91 + this PR's additions)
• npm test — 16/16 passing, tsc --noEmit clean
• Two rounds of automated code review (task-level + whole-branch), both approved with fixes applied
• Manual backoffice smoke test (all four block types, stylesheet loading) — not run this cycle, no browser access available; recommend running before a release tag

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