Parse GitHub timestamps with the invariant culture in GitHubClient #723

GitHubClient parses ISO-8601 timestamps with the current culture and no RoundtripKind, unlike its three sibling clients. On a machine whose culture is not invariant, issue and pull-request timestamps can parse differently or fail, and the value silently becomes null. Found in the 2026-09-27 repository review.

Context

src/Infrastructure/Backlog.Infrastructure.GitHub/GitHubClient.cs line 400:

&& DateTimeOffset.TryParse(value.GetString(), out var parsed)

The three other copies of the same helper pass CultureInfo.InvariantCulture, DateTimeStyles.RoundtripKind:

• src/Infrastructure/Backlog.Infrastructure.GitHub/CopilotUsageClient.cs line 259
• src/Infrastructure/Backlog.Infrastructure.GitHub/GitHubActivityClient.cs line 1055
• src/Infrastructure/Backlog.Infrastructure.Claude/ClaudeUsageClient.cs line 393

Fix the parse in GitHubClient.cs to match. While there, collapse the four Timestamp(JsonElement, string) helpers (and the three identical Rfc3339 helpers at GitHubActivityBaselineClient.cs line 166, GitHubActivityClient.cs line 1037 and ClaudeUsageClient.cs line 366) into one internal helper so the next copy cannot drift. Add a test in tests/Backlog.Infrastructure.GitHub.UnitTests that runs the parse under a culture such as de-DE and asserts the same instant comes back.

Route through delivery:flow-code per CLAUDE.md.