Carry the source file's using directives into the generated file

[virzak]

Problem

Documentation comments are copied into the generated file verbatim, but a cref in one is resolved against the file it lands in, not the file it was written in. The generated file carries none of the source file's using directives, so every cref which relied on one goes unresolved:

warning CS1574: XML comment has cref attribute 'Task' that could not be resolved
warning CS1574: XML comment has cref attribute 'ArgumentException' that could not be resolved

For a project which treats warnings as errors - which is most of the ones worth adopting this - that is a build failure, not a nit. Found while converting SSH.NET, where a single converted method produced seven of them.

Fix

Emit the directives the source file declared, on the side of the namespace they were declared on:

// <auto-generated/>
#nullable enable

using System.Text;

namespace Test
{
    using System.Net.Sockets;

    public partial class Class
    {

The distinction matters: a directive written inside a namespace may name another namespace relatively, and would not resolve if hoisted above it. The generated code itself is fully qualified and needs none of them, so nothing here changes what the code means - only what the documentation can refer to.

Also in this change: a <returns> element is dropped when the synchronized method returns nothing, because the task it describes is no longer returned. A <returns> describing a value which survives is kept - it is imprecise about the task it mentions, but dropping it would say less than saying it imprecisely. Rewriting the prose is not something the generator can do.

Verification

First commit records both behaviours as they are today, so the second commit's diff is exactly what changed. 19 existing snapshots pick up the directives their sources declared; no generated code changed.

Full suite passes on both target frameworks (338 and 297) and all three Roslyn variants build.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.92%. Comparing base (9205073) to head (fbda0c4).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...p.SyncMethodGenerator/SyncMethodSourceGenerator.cs	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   93.82%   93.92%   +0.09%     
==========================================
  Files           9        9              
  Lines        1522     1563      +41     
  Branches      352      360       +8     
==========================================
+ Hits         1428     1468      +40     
  Misses         25       25              
- Partials       69       70       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.