feat: fill test and documentation coverage gaps (#58)

* docs: add coverage-gaps design doc

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: add coverage-gaps implementation plan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test: add Merge unit tests for AsyncDataTable

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test: add Merge unit tests for AsyncDataTable

* test: add Compute unit tests for AsyncDataTable

* test: add Compute unit tests for AsyncDataTable

* test: add LoadDataRow unit tests for AsyncDataTable

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test: add LoadDataRow unit tests for AsyncDataTable

* test: add BeginInit, BeginLoadData, Reset unit tests for AsyncDataTable

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test: add BeginInit, BeginLoadData, Reset unit tests for AsyncDataTable

* docs: add XML I/O cookbook page

* docs: add Merge and row versioning cookbook page

* docs: add Compute, constraints, and relations cookbook page

* docs: add DataRowVersion.Proposed example to merge-and-row-versioning cookbook

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: MarcelRoozekrans

docs/plans/2026-04-17-coverage-gaps-design.md

@@ -0,0 +1,62 @@
+# Design: Fill Test & Documentation Coverage Gaps
+
+**Date:** 2026-04-17  
+**Status:** Approved  
+**Branch:** `feat/coverage-gaps`
+
+---
+
+## Overview
+
+The project is feature-complete at v1.1.4 but has identified gaps in test coverage and Docusaurus documentation for several delegated ADO.NET features. This document describes what will be added and how.
+
+---
+
+## Scope
+
+### Tests (new test class in `System.Data.Async.DataSet.Tests`)
+
+| Feature | What to test |
+|---------|-------------|
+| `Merge()` | `MissingSchemaAction.Add`, `MissingSchemaAction.Ignore`, `MissingSchemaAction.Error`; `preserveChanges=true` vs `false`; merge into empty table; merge with overlapping rows |
+| `Compute()` | Aggregate expressions (`Sum`, `Count`, `Avg`, `Min`, `Max`) on a plain `AsyncDataTable`; filter expressions; `DBNull` handling |
+| `LoadDataRow()` | `LoadOption.OverwriteChanges`, `LoadOption.PreserveChanges`, `LoadOption.Upsert`; bool overload (accept=true/false) |
+| `BeginInit/EndInit` | Call sequence does not throw; columns added between Begin/End are visible after EndInit |
+| `BeginLoadData/EndLoadData` | Row events are suppressed during load; constraints are re-evaluated after EndLoadData |
+| `Reset()` | Table is empty after Reset; columns are cleared |
+
+All tests go in a new file `tests/System.Data.Async.DataSet.Tests/AsyncDataTableAdvancedTests.cs`.
+
+### Docusaurus Docs (new pages in `website/docs/cookbook/`)
+
+| File | Content |
+|------|---------|
+| `xml-io.md` | Async `WriteXmlAsync` / `ReadXmlAsync` round-trip; `WriteXmlSchemaAsync`; reading from a file path; streaming patterns |
+| `merge-and-row-versioning.md` | `Merge()` with each `MissingSchemaAction`; `preserveChanges`; reading `DataRowVersion.Original` vs `Current` vs `Proposed` via `AsyncDataRow[name, version]`; `GetChanges()` workflow |
+| `compute-constraints-relations.md` | `Compute()` aggregate expressions on a plain table; adding `UniqueConstraint` and `ForeignKeyConstraint` manually; adding `DataRelation` and navigating parent/child rows |
+
+All pages follow the existing cookbook format: frontmatter with `sidebar_position` and `title`, H2 sections per scenario, C# code blocks, brief prose.
+
+---
+
+## Architecture
+
+No new source code. All changes are:
+- One new test file (tests only, no production code changes)
+- Three new Markdown pages (docs only)
+
+No API changes, no breaking changes, no NuGet version bump required.
+
+---
+
+## Testing Strategy
+
+- New tests use the existing `xunit` + `FluentAssertions` stack
+- Tests are synchronous where the feature is synchronous (Merge, Compute, LoadDataRow) — async `AddAsync`/`AcceptChangesAsync` used for setup only
+- All existing 228 + 109 tests must continue to pass
+
+---
+
+## Delivery
+
+Single branch `feat/coverage-gaps`, single PR to `main`.