[release/9.0] [OpenAPI] Use invariant culture for TextWriter

[martincostello]

Use culture-invariant TextWriter implementation for OpenAPI.

Port of #62193 for .NET 9.

Description

To avoid any accidental changes to SignalR's use of Utf8BufferTextWriter I've removed the use of the writer pooling for OpenAPI and instead a new instance is created every time.

Fixes #60628

Customer Impact

Developers run into this issue when hosting under non-en cultures, but no workarounds are available.

Regression?

• Yes
• No (Bug has existed throughout 9.0 previews; not a regression from an earlier serviced release.)

Risk

• High
• Medium
• Low

Change is isolated to the OpenAPI serialization path and uses an existing overload; no impact on runtime behavior or other subsystems.

Verification

• Manual
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[captainsafia]

@martincostello I updated the issue with the servicing template in case you'd like to review it and mark the PR as ready for review.

[Copilot]

Pull Request Overview

This PR addresses a culture-invariant formatting issue in OpenAPI document generation. When hosting applications under non-English cultures (like fr-FR), the OpenAPI serialization was producing incorrect JSON with locale-specific number formatting (e.g., using commas instead of dots for decimals).

• Introduces a new constructor for Utf8BufferTextWriter that accepts an IFormatProvider
• Updates the OpenAPI endpoint handler to use CultureInfo.InvariantCulture for serialization
• Refactors test approach from direct document serialization to HTTP endpoint testing

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Utf8BufferTextWriter.cs	Adds constructor overload accepting IFormatProvider for culture-invariant formatting
OpenApiEndpointRouteBuilderExtensions.cs	Updates OpenAPI serialization to use invariant culture and creates new writer instance instead of pooled one
OpenApiDocumentIntegrationTests.cs	Simplifies tests to use HTTP client instead of direct document service
Program.cs	Adds middleware to set French culture for testing culture-invariant behavior
TestController.cs	Adds test endpoint with float property to verify decimal formatting
Multiple snapshot files	Updates test snapshots with new server URL and endpoint entries

[martincostello]

@captainsafia Thanks, done. I wanted to make sure you were happy with the changes/approach first before trying to guide it through shiproom.

[captainsafia]

LGTM!

servers change to snapshots is expected since we move from resolving the document via the service to via the endpoint.

New test scenario matches what we have in main.

[danmoseley]

@captainsafia tactics approved but asked the usual question about whether we scanned for variants (other instances)?

[wtgodbe]

Holding this for October at this point

[wtgodbe]

/azp run