ci: add DOM-surface regression integration test

[gavinbarron]

Description

Adds a DOM-surface regression integration test to catch binary/source breaking changes that a change to kiota''s generation logic could introduce into downstream SDKs (notably the Microsoft Graph SDKs), mirroring the surface-area validation the Graph SDK repos already run on spec changes.

How it works

Per language, the new workflow:

1. Downloads a single OpenAPI description (Microsoft Graph beta, already in the IT matrix).
2. Generates a baseline DOM export (kiota-dom-export.txt) using the published Microsoft.OpenApi.Kiota NuGet tool.
3. Generates a current DOM export using the locally built kiota.
4. Produces a git format-patch style patch between the two.
5. Feeds the patch to microsoftgraph/kiota-dom-export-diff-tool/tool with fail-on-removal: true (pinned to a commit SHA); on PRs it posts the explanations as a comment.

No generator code change is required — the export is enabled via the existing Generation:ExportPublicApi flag (env var KIOTA_Generation__ExportPublicApi=true).

Files

• it/generate-dom-export.ps1 — emit the DOM export for one generator/language/spec.
• it/compare-dom-export.ps1 — install the baseline tool, build baseline + current exports, emit the patch.
• .github/workflows/surface-area-tests.yml — csharp + java × Graph beta matrix, analyze + comment + artifacts + branch-policy aggregator.
• it/Readme.md, CHANGELOG.md — docs.

Initial scope

csharp and java against Graph beta, designed to expand to more languages/specs later.

Notes / caveats

• The baseline is the last published release, so the diff reflects every change since that release, not only this PR''s delta. Acceptable for catching regressions; a base-branch comparison mode can be added later.
• The patch is built with git format-patch -1 HEAD --minimal -U0 -w (not git diff --no-index) because the diff tool''s parser requires zero-context bodies and a trailing -- signature footer (DOM exports contain --> inheritance markers). Validated against the tool''s actual cleanup/parse algorithm.

Validation

Validated locally end-to-end: the env var enables the export, the published baseline tool installs and runs, both exports generate, and the produced patch parses cleanly through the diff tool''s algorithm with zero crash-inducing lines.

Closes #7858

[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall coverage in the gavinbarron/dom-surf... branch is 71%. Coverage data for the main branch is not yet available.

Show a code coverage summary of the most covered files.

File	main	gavinbarron/dom-surf... a10d3d0	+/-
/home/runner/wo...guageRefiner.cs	—	98%	—
/home/runner/wo...criptRefiner.cs	—	98%	—
/home/runner/wo...MethodWriter.cs	—	97%	—
/home/runner/wo...MethodWriter.cs	—	96%	—
/home/runner/wo...MethodWriter.cs	—	96%	—
/home/runner/wo...MethodWriter.cs	—	95%	—
/home/runner/wo...rs/GoRefiner.cs	—	93%	—
/home/runner/wo...KiotaBuilder.cs	—	90%	—
/home/runner/wo...ationService.cs	—	89%	—
/home/runner/wo...xGenerator.g.cs	—	72%	—

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[baywet]

The problem you'll have with beta is that API version allows breaking changes.
So from one week to another, you'll get noise not because of a Kiota regression, but because of the source.
I recommend to use v1 instead