Add missing CultureInfo.InvariantCulture use to LogValuesFormatter

[stephentoub]

Fixes #116979

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[Copilot]

Pull Request Overview

This PR ensures that LogValuesFormatter uses CultureInfo.InvariantCulture when formatting values and adds a corresponding test to validate culture-invariant output.

• LogValuesFormatter formatting methods now explicitly pass CultureInfo.InvariantCulture for placeholder indices and enumerable elements.
• A new unit test FormatMessage_UsesInvariantCulture has been added to LoggerExtensionsTest.cs to verify behavior under a non-default culture.
• Imported System.Globalization where needed.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/Microsoft.Extensions.Logging.Abstractions/src/LogValuesFormatter.cs	Use CultureInfo.InvariantCulture in ToString calls for consistent output
src/libraries/Microsoft.Extensions.Logging/tests/Common/LoggerExtensionsTest.cs	Added test FormatMessage_UsesInvariantCulture and imported System.Globalization

Comments suppressed due to low confidence (1)

src/libraries/Microsoft.Extensions.Logging.Abstractions/src/LogValuesFormatter.cs:69

• Ensure the System.Globalization namespace is imported at the top of this file so that CultureInfo.InvariantCulture resolves correctly and the code compiles without errors.

                    vsb.Append(_valueNames.Count.ToString(CultureInfo.InvariantCulture));

[tarekgh]

LGTM, Thanks!