fix(D011): forwarded queries use pick, the mock sources reply with their own @context

• The forward stubs expected attrs= for the forwarded projection. attrs is also an entity SELECTION (an entity with none of the attributes does not match), so a broker forwards the projection-only pick= instead (TS 104-175 clause 4.21); the stubs now expect pick.
• The mock Context Source replies carried no @context. A forwarded response is interpreted with the context that travels with it (TS 104-176 clause 6.3.5), so 'speed' expanded with core only and was dropped on merge. The stub replies now load vehicle-simple-attributes-with-context.json, vehicle-simple-attributes-second-with-context.json (new, stub-reply only; the shared request fixtures are untouched) and vehicle-speed-attribute.json (now with the suite's compound @context; it is used only as a stub reply).
• Query Entities Via POST sent Content-Type application/ld+json with the @context in a Link header and none in the body (TS 104-176 clause 6.2.4). For ld+json the @context now goes in the body; plain json keeps the Link.
• D011_02_exc: the registration's operations per method - queryBatch for POST /entityOperations/query, queryEntity for GET /entities.

Tested - Against coraine, DistributedOperations (101 TCs): develop 60 pass / 41 fail, this branch 74 / 27 - the 14 D011 TCs that failed now pass, nothing else changed. QueryEntities (the users of Query Entities Via POST): 236 / 238 on both (019_26_01/04, unrelated).

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