fix: revert ObservableStatus to a strict (non-union) type (#750)

* test: type-check the test suite in CI to catch type regressions

Adds tsconfig.test.json (type-checks src + test) and runs it in the
type-check workflow. The test suite already exercises hooks the way
consumers do (reading .data off results), so a breaking change to the
public types now fails CI at compile time.

Also fixes pre-existing test-only type issues surfaced by turning the
check on: unused imports, node globals (require/global/Buffer) via the
node types, and a FunctionComponent children prop annotation.

* fix: revert ObservableStatus to a strict (non-union) type

4.2.4 shipped ObservableStatus<T> as a discriminated union (data is
T | undefined unless narrowed on status), which broke the documented
pattern of reading .data directly across every data hook. Revert the
type to the previous strict shape (data: T) so this ships as a patch.

The genuinely-optional cases stay correctly typed at the hook level
(useFirestoreDocData/DocDataOnce return T | undefined, useUser returns
User | null, useDatabaseListData returns T[] | null), which is where
the undefined belongs rather than on ObservableStatus itself.

The internal SuspenseSubject status builders keep an as-cast since the
in-flight loading value is internally T | undefined.

* docs: regenerate reference docs for ObservableStatus type change

ObservableStatus is now an interface again rather than a discriminated
union type-alias, so the generated reference docs drop the per-status
sub-interface pages and retarget links from type-aliases/ to interfaces/.

* test: strengthen the type guard with typed object-data and doc-data reads

Per review: the guard did not exercise useDatabaseObjectData, and the
existing useFirestoreDocData tests use <any> so their .data reads are
untyped. Add a useDatabaseObjectData test with a typed .data read (data
is T, so it fails to compile if ObservableStatus regresses to a union)
and a concretely-typed useFirestoreDocData read.

Author: tyler-reitz

.github/workflows/test.yaml

@@ -100,6 +100,8 @@ jobs:
         run: npm install --no-save react@${{ matrix.react }} react-dom@${{ matrix.react }} @types/react@${{ matrix.react }} @types/react-dom@${{ matrix.react }}
       - name: Type check
         run: npx tsc --noEmit
+      - name: Type check tests
+        run: npx tsc --noEmit -p tsconfig.test.json
   publish:
     runs-on: ubuntu-latest
     name: Publish (NPM)