Make dotnet-watch tests conditional in PR validation

[Copilot]

Dotnet-watch tests currently run on every PR, consuming Helix capacity even for unrelated changes. This change applies existing conditional test filtering while retaining full coverage on non-PR builds.

Changes

• Define the DotnetWatch scope

  • Covers all six watch-owned test projects.
  • Activates for CLI, watch, ProjectTools, Razor, Static Web Assets, WASM, and related test-asset changes.
  • Uses RunAlways=CI to preserve full main and release validation.

• Keep test selection centralized

  • Removes the redundant dotnet-watch mapping from the targeted-test fallback table.
  • Leaves test/ConditionalTests.props as the source of truth for PR filtering and targeted local validation.

• Fixes Make dotnet-watch test execution conditional in PR validation #55462

[azure-pipelines]

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