Bump Microsoft.OpenApi from 3.8.0 to 3.9.0

[dependabot]

Updated Microsoft.OpenApi from 3.8.0 to 3.9.0.

Release notes

Sourced from Microsoft.OpenApi's releases.

3.9.0

3.9.0 (2026-07-15)

Features

• adds support for anchor and id external resolution (1591007)
• schema: resolve bare $dynamicRef via $dynamicAnchor index (#​2913) (eacc2fc)
• support relative URI resolution in $dynamicRef (#​2928) (#​2945) (821053b)

Bug Fixes

• adds explicit error message for invalid json pointers (63fc55d)
• adds explicit error message for invalid json pointers (bc93efe)
• default mapping is not being serialized with the correct shape (fe4a25f)
• differentiate unset value from null value in OpenApiSchema.Const (#​2936) (07b525f)
• handle nullability more accurately during serialization for 3.0/2.0 (#​2933) (0ace243)
• validate required properties of security scheme before serialization (#​2952) (f31b192)

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[baywet]

@copilot update the faulty unit test

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

@copilot update the faulty unit test

Done. Updated the 4 resource baseline files (TripService.OpenApi.{json,yaml}, Multiple.Schema.OpenApi.{json,yaml}) and 2 inline test expectations in OpenApiSchemaGeneratorTests.cs to reflect the nullable serialization change in Microsoft.OpenApi 3.9.0 for OpenApi3_0: nullable: true is now represented as { "enum": [null] } within the anyOf array. All 1279 tests pass (commit f7c8e84).