refactor(compiler-cli): validate @content block placement

Adds validation to verify that `@content` blocks are only used as direct
children of foreign components.

Specifically:
- Defines a new compile diagnostic code `INVALID_CONTENT_PLACEMENT = 8026`.
- Updates `ForeignComponentFeatureAnalyzer` to traverse content blocks and
  report `INVALID_CONTENT_PLACEMENT` diagnostics if they are placed
  incorrectly.
- Removes the raw error thrown during ingestion in
  `packages/compiler/src/template/pipeline/src/ingest.ts`.
- Adds integration tests in `template_typecheck_spec.ts`.

goldens/public-api/compiler-cli/error_code.api.md

@@ -81,6 +81,7 @@ export enum ErrorCode {
     INLINE_TYPE_CTOR_REQUIRED = 8901,
     INTERPOLATED_SIGNAL_NOT_INVOKED = 8109,
     INVALID_BANANA_IN_BOX = 8101,
+    INVALID_CONTENT_PLACEMENT = 8026,
     LET_USED_BEFORE_DEFINITION = 8016,
     LOCAL_COMPILATION_UNRESOLVED_CONST = 11001,
     LOCAL_COMPILATION_UNSUPPORTED_EXPRESSION = 11003,