Restore net45 compatibility in ResizingUtil.AppendQuery to unblock Windows CI build

[Copilot]

The failing GitHub Actions Windows build was caused by a net45 compile error in ResizingClient: string.IndexOf(char, StringComparison) is not available for that target. This PR applies a minimal compatibility fix in the URL query-separator path.

• Root cause

  • AppendQuery used a newer IndexOf overload incompatible with net45, breaking ResizingClient compilation in the CI job.

• Change made

  • Replaced the incompatible overload with a net45-safe check while preserving behavior (detect existing ? and choose & vs ?).

• Scope

  • One-line production code change in source/ResizingClient/ResizingUtil.cs.
  • No API shape changes; URL formatting logic remains the same.

// before
var separator = urlWithoutFragment.IndexOf('?', StringComparison.Ordinal) >= 0 ? "&" : "?";

// after
var separator = urlWithoutFragment.IndexOf('?') >= 0 ? "&" : "?";

[Copilot]

Copilot review overview

🟢 Approval recommended

The compatibility fix is minimal and preserves existing behavior.

Review effort: Lite
Findings: None

What changed in this PR

Restores net45 compatibility in ResizingUtil.AppendQuery without changing URL behavior.

Changes:

• Replaces the unsupported IndexOf overload.
• Preserves query-separator handling.

File	Summary
source/​ResizingClient/​ResizingUtil.cs	Updates AppendQuery for net45 compatibility.

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