feat(adr-0027): add @extend_schema to Xblock v1 viewset

[taimoor-ahmed-1]

Summary

Follow-up from #38773 review comment: Xblock v1 previously had no OpenAPI schema decoration, which was flagged as a pre-existing gap and deferred to a separate PR. This closes that loop.

• Adds drf-spectacular @extend_schema to all five XblockViewSet actions (create, retrieve, update, partial_update, destroy) so the endpoints appear with proper request/response bodies, path/query parameters, and error codes in the generated OpenAPI 3.x schema.
• Documents the ADR 0036 ?view=minimal preset in the schema (enum=["minimal"]) so consumers (Swagger UI, generated SDK clients) can discover it.
• Documents the legacy ?fields= selector (graderType / ancestorInfo / customReadToken) as deprecated=True. Its semantics are pass-through / type-of-response, not the ADR 0036 CSV subset convention — new callers should use ?view=minimal instead.
• Shared parameter and response building blocks are declared at module scope to keep the per-action decorators readable.

No behavior change — this is a documentation-only PR from the runtime's perspective.

Test plan

• ruff check passes on the modified file
• python -c "import ast; ast.parse(open(...).read())" parses cleanly
• Existing xblock v1 tests continue to pass in CI
• manage.py spectacular (or equivalent) picks up the new decorators without warnings

🤖 Generated with Claude Code

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