Handle runtimeconfig.dev.json without additionalProbingPaths

[tmat]

Fix issue blocking dotnet/sdk#53715

Change by Copilot.

DotnetTestHostManager.GetTestHostPath read 'additionalProbingPaths' from
runtimeconfig.dev.json using GetProperty (NETCOREAPP) and a dictionary indexer
(NETFRAMEWORK), both of which throw when the property is absent. The .NET SDK
now emits a runtimeconfig.dev.json for net6.0+ Debug builds that may only carry
other runtime options (e.g. Hot Reload switches), causing a KeyNotFoundException
that aborted the test run. Use TryGetProperty / TryGetValue and skip the probing
path lookup when the property is missing.

Adds a filesystem integration test covering a dev config without
additionalProbingPaths on both the NETCOREAPP and NETFRAMEWORK code paths.

[Copilot]

Pull request overview

Improves DotnetTestHostManager robustness when parsing *.runtimeconfig.dev.json by tolerating Debug-time SDK-emitted files that omit runtimeOptions.additionalProbingPaths, preventing test runs from aborting due to missing JSON properties.

Changes:

• Update GetTestHostPath to use TryGetProperty / TryGetValue and skip probing-path resolution when additionalProbingPaths is absent.
• Add a filesystem integration test validating GetTestHostProcessStartInfo succeeds when runtimeconfig.dev.json exists but does not define additionalProbingPaths.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Microsoft.TestPlatform.TestHostProvider/Hosting/DotnetTestHostManager.cs	Makes runtimeconfig.dev.json probing-path parsing tolerant of missing additionalProbingPaths on both NETCOREAPP and NETFRAMEWORK code paths.
test/Microsoft.TestPlatform.TestHostProvider.UnitTests/Hosting/DotnetTestHostManagerFilesystemIntegrationTests.cs	Adds regression coverage ensuring resolution falls back to testhost.dll next to the test assembly when probing paths are not provided.