Improve span details percentage calculation

[JamesNK]

Description

Spans could have zero duration. This will cause NaN values (floating point divide by zero), which when used in Blazor view will product NaN% strings.

This wouldn't break the page - the browser will ignore it - but it will avoid browser writing CSS warning.

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire-docs issue (consider using one of the following templates):
      • New (or update) doc-idea template
      • New breaking-change template
      • New diagnostic template
  • No

[Copilot]

Pull Request Overview

This PR fixes a bug where spans with zero duration would cause NaN percentage values in the dashboard's waterfall visualization. The fix introduces a helper method to safely calculate percentages by returning 0 when the total duration is 0, preventing division by zero errors.

Key changes:

• Added a CalculatePercent helper method to handle division by zero scenarios
• Enhanced test helper methods to support creating spans and logs with zero duration
• Added comprehensive test coverage for the zero duration scenario

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/Aspire.Dashboard/Model/Otlp/SpanWaterfallViewModel.cs	Added CalculatePercent helper method and replaced direct percentage calculations to prevent NaN values
tests/Aspire.Dashboard.Tests/Model/SpanWaterfallViewModelTests.cs	Added test case to verify zero duration spans are handled correctly
tests/Shared/Telemetry/TelemetryTestHelpers.cs	Enhanced test helpers with additional parameters and utility methods for better test support