Add text/plain response support to IMessageResponseBuilder

[davidkallesen]

Summary

• Unblock OpenAPI text/plain clients from the REST generator
• Return raw text bodies verbatim without JSON deserialization
• Refresh atc-coding-rules to 1.0.1 and bump analyzer packages

Changes

✨ Features

• Add AddSuccessTextResponse(HttpStatusCode) to the builder
• Add AddErrorTextResponse(HttpStatusCode) to the builder
• Bypass the contract serializer for registered text status codes
• Mirror AddTypedResponse<T> empty-body semantics (null for whitespace)

📝 Documentation

• Add "Plain Text Responses" usage section to README
• Update IMessageResponseBuilder block in the API reference
• Add Plain Text Responses entry to the Features list and TOC

📦 Dependencies

• Bump Atc.Analyzer 0.1.22 -> 0.1.23
• Bump Meziantou.Analyzer 3.0.7 -> 3.0.50
• Bump SonarAnalyzer.CSharp 10.19.0 -> 10.24.0
• Bump Microsoft.SourceLink.GitHub 10.0.103 -> 10.0.203
• Bump Microsoft.Testing.Extensions.CodeCoverage 18.4.1 -> 18.6.2
• Bump Microsoft.Testing.Extensions.TrxReport 2.1.0 -> 2.2.1

🔥 Removal

• Drop Asyncify package reference and editorconfig section
• Drop SecurityCodeScan.VS2019 package reference

🔧 Configuration

• Sync all .editorconfig files to atc-coding-rules v1.0.1
• Replace external rule-URL comments with inline descriptions

Notes

• Atc.Rest.Client targets netstandard2.0, so default interface
  methods are unavailable - the new methods are plain additions
• Additive interface change; external implementers of
  IMessageResponseBuilder must add the two methods on upgrade
• Charset is honored via ReadAsStringAsync; no extra handling needed
• Enables the generator to switch EndpointPerOperation text clients
  from inline emission to the canonical builder pathway