Upgrade Microsoft.OpenApi to 3.x when moving to .NET 11 #301

Follow-up from #279 / #300 (chore(deps): ignore Microsoft.OpenApi major updates and document the 2.x pin).

Problem

Microsoft.OpenApi is pinned to 2.x (2.7.5, the CVE-2026-49451 / GHSA-v5pm-xwqc-g5wc fix for 2.x). The 3.x line cannot be used on .NET 10: Microsoft.AspNetCore.OpenApi on .NET 10 caps Microsoft.OpenApi below 3.0.0, and its source generator fails with CS0200 on IOpenApiMediaType.Example against 3.x.

Dependency ranges, from the published nuspecs on nuget.org:

There is no stable 11.0.0 yet. The project is staying on .NET 10 for now, so #300 added a Dependabot ignore for Microsoft.OpenApi major updates (.github/dependabot.yml, nuget block). That ignore has no expiry.

Scope

Upgrade Microsoft.OpenApi to a 3.x release once the project moves to .NET 11 (and Microsoft.AspNetCore.OpenApi 11.x is stable).

• Use 3.5.4 or later. 3.x versions up to 3.5.3 are affected by CVE-2026-49451; 3.5.4 is the patched release.
• Remove the ignore for Microsoft.OpenApi in .github/dependabot.yml and the pin comment in TaskFlow.Api/TaskFlow.Api.csproj.
• Re-check that /openapi/v1.json and npm run gen:api output are unchanged or intentionally changed.
• Link with chore: upgrade Asp.Versioning.Mvc to v10.0.0 #165 (Asp.Versioning.Mvc v10), resolved 2026-10-30 from the nuspec: Asp.Versioning.OpenApi 10.2.3 depends on Microsoft.OpenApi [2.7.5, 3.0.0), so adopting it on .NET 10 would block this upgrade until Asp.Versioning ships a release that accepts Microsoft.OpenApi 3.x. chore: upgrade Asp.Versioning.Mvc to v10.0.0 #165 is therefore on hold. Check for such a release before starting this issue.
• Part of [Tracking] Move to .NET 11 #372 (Move to .NET 11).

Blocked by

The decision to move the project to .NET 11. If that decision is never made, this issue can be closed together with the ignore rationale.

_{Created with Claude Code following #300}