Fix culture-dependent date formatting and cross-platform line endings in OpenAPI document generation

[Copilot]

Fix culture-dependent date formatting and cross-platform line endings in OpenAPI document generation

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Use CultureInfo.InvariantCulture and explicit \n line endings so OpenAPI documents are identical regardless of locale or OS.

Description

OpenAPI acceptance tests fail on non-en-US machines (e.g. Dutch locale) because deprecation/sunset dates are formatted via CultureInfo.CurrentCulture with {1:d}, producing locale-specific output (1/1/2026 vs 1-1-2026). Additionally, StringBuilder.AppendLine() emits \r\n on Windows and \n on Linux, causing cross-platform mismatches against the expected JSON fixture.

• OpenApiDocumentDescriptionOptions.cs — CultureInfo.CurrentCulture → CultureInfo.InvariantCulture in DefaultDeprecationNotice and DefaultSunsetNotice
• ApiExplorerTransformer.cs — Replace AppendLine() with explicit Append('\n') in Markdown generation for deterministic output
• OpenApiDocumentDescriptionOptionsTest.cs — Update expected dates to invariant format (02/08/2026)
• Content/v1.json — Update dates and \r\n → \n to match corrected output

// Before: output varies by locale
return string.Format( CultureInfo.CurrentCulture, deprecatedNoticeFormat, participle, when );

// After: deterministic across all locales
return string.Format( CultureInfo.InvariantCulture, deprecatedNoticeFormat, participle, when );

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