Update Microsoft.OpenApi packages to 2.9.0 from OpenAPI.NET PR #2927

[Copilot]

Bumps Microsoft.OpenApi and Microsoft.OpenApi.YamlReader from 2.0.0 to 2.9.0, built from the fix/null-enums-to-v2 branch (PR microsoft/OpenAPI.NET#2927), which fixes nullable enum serialization in OpenAPI 3.0 output.

Description

• eng/Versions.props: MicrosoftOpenApiVersion and MicrosoftOpenApiYamlReaderVersion 2.0.0 → 2.9.0
• NuGet.config: adds local-packages relative feed for the locally built nupkgs
• local-packages/: Microsoft.OpenApi.2.9.0.nupkg + Microsoft.OpenApi.YamlReader.2.9.0.nupkg (temporary; to be replaced by official NuGet release once upstream PR merges)
• Snapshot fix (schemas-by-ref, OpenAPI 3.0): nullable types are now serialized as type + nullable: true instead of anyOf: [{nullable: true}, {type schema}], which is more spec-compliant

All OpenAPI tests pass: 790 (main), 18 (source generators), 3 (build).

Customer Impact

Picks up the upstream fix for incorrect nullable enum serialization when targeting OpenAPI 3.0 output.

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

Dependency version bump with one snapshot update. No production logic changes in this repo.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

---

When servicing release/2.1

• Make necessary changes in eng/PatchConfig.props