fix(hottesting): a feed that reads a service is a required input, whatever else it reads - #3287

GitHub Issue (If applicable): closes #3283

PR Type

• Bugfix

What is the current behavior?

A feed that reads both a constructor service and another feed of the model is classified as derived, so it is optional in {Model}Mock and left out of Empty. Two common shapes are affected:

• a search over a query state: Query.SelectAsync((query, ct) => Service.Search(query, ct));
• a list with a selection: ListFeed.Async(...).Selection(SelectedTask).

Left unset, the member runs its real logic over the null-injected service. Create(), and any mock that does not set it, shows its error.

What is the new behavior?

• A feed that reads a service is a required input, whatever else it reads. Empty covers it, and the compiler asks for it in every mock.
• This matches spec 013 §7: the required members are exactly the service-dependent feeds. A derived member is optional because its real logic recomputes over the mocked inputs, and that cannot hold when the logic reads a service.
• Both paths agree:
  • the shared classification ranks a parameter before a feed;
  • the MVUX generator emits both OnParameter and OnFeed for such a member;
  • the metadata path treats a member as derived only when every [FeedDependency] on it names a feed.
• A service is a parameter of one of the model's constructors. A lambda parameter with the same name doesn't count.

Reviewer's guide

• Start here: FeedDependencyAnalysis.ClassifyFeedMember, where the two checks swap order, then FeedsMockGenerator.ReadFeedDependencies.
• Reverses a pinned rule: fix(hottesting): declare a mock member once when it has several dependencies #3193 pinned "a member declared both input and derived is derived" in When_MemberIsDeclaredBothInputAndDerived_Then_MockTreatsItAsDerivedLikeTheSourcePath. That test is renamed ..._Then_MockRequiresItLikeTheSourcePath and now asserts the opposite.
• Tests that prove it: the first commit, 0a776dce0, adds tests only, and all of them fail on main.
  • Given_GeneratedMock.When_CreateDefault_Then_AFeedReadingAServiceAndAnotherFeedIsEmptyToo (fixture SearchModel): on main the list's first message carries the NullReferenceException from the real Search.
  • Given_GeneratedMock.When_FeedReadsAServiceAndAnotherFeed_Then_MockRequiresIt.
  • Given_FeedsMockGenerator: a search, a list with a selection, and the hand-declared mix above.
• Review fix: 48780d9b9. A lambda parameter named like a constructor parameter made a derived feed required; When_DerivedFeedHasALambdaParameterNamedLikeACtorParameter_Then_ItStaysOptional failed before it. The same commit adds the OnFeed emission and the attribute remarks.
• Docs commits: 4746e8fd3 and 664fa6563: testing.md and spec 013 (architecture, implementation, history v20).
• Unrelated, one line: 02ff6a312 replaces a hard tab in spec 026 that came from main (fix(reactive): MVUX hot reload of a command's parameters crashes or loses list updates #3296), so the docs validation passes.

PR Checklist

• Tested code with current supported SDKs: unit tests only (Uno.HotTesting.Reactive.Tests 44 pass; Uno.Extensions.Reactive.Tests 1655 pass, 19 skipped; Uno.Extensions.Core.Tests 61 pass). UI and runtime tests were not run locally.
• Docs have been added/updated.
• Unit Tests for the changes have been added: 6 new, 1 updated.
• Contains NO breaking changes: see below.
• Associated with an issue.

Breaking change: a mock that left such a member unset no longer compiles, because the member is now required. Setting it, or starting from {Model}Mock.Empty with { ... }, fixes the call site. Because the change is breaking, it should not be backported to release/stable/8.0.

Other information

• A list with a selection is now required and covered by Empty. Until fix(hottesting): mock a list with a selection #3247 merges, though, its swap still does not reach its readers, so it keeps showing the real loader's error, as before. testing.md names only the search shape for now.
• Accepted simplification: every constructor parameter counts as a service, including one a mock doesn't leave null (a messenger, a value type). Telling them apart would put mocking rules into the shared classification.
• On the metadata path, the change reaches a model once the app is rebuilt with this version of the MVUX generator.
• Spec 013's history entry is numbered v20, as in fix(hottesting): mock a list with a selection #3247 and fix(hottesting): Create takes the services a view model reads while it is built #3285. Whichever merges later renumbers.
• Independent of fix(hottesting): Create takes the services a view model reads while it is built #3285. Both touch FeedDependencyAnalysis.cs and FeedsMockGenerator.cs, in different methods. Only the spec files will conflict.

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