Don't abort the run on an unpaired surrogate in a test name

[Azat Mukhametshin (azat-msft)]

Fixes #16512.

A test run on .NET Framework aborts completely when a single test name, error message or stack trace contains an unpaired UTF-16 surrogate:

The active test run was aborted. Reason: The JSON value could not be converted to
Microsoft.VisualStudio.TestPlatform.ObjectModel.TestCase.
---> System.InvalidOperationException: Cannot read incomplete UTF-16 JSON text as string with missing low surrogate.

This is easy to hit without meaning to. xUnit v2 truncates long theory arguments by UTF-16 length, so any emoji in theory data can be cut in half. Every result after the bad message is lost.

Why it broke in 18.11

.NET Framework testhosts serialize with Jsonite, which escapes every surrogate code unit on its own, so an emoji goes on the wire as \uD83D\uDE00. A truncated one goes as a lone \uD83D. Since 18.11 the .NET runner reads those messages with System.Text.Json, which rejects unpaired surrogates by design.

Before 18.11 the runner used Newtonsoft, which wrote non-ASCII literally, so the UTF-8 encoder on the channel silently turned the lone surrogate into U+FFFD and nothing ever noticed.

The change

Reader (JsonSurrogates.cs, .NET only). Unpaired surrogate escapes are rewritten to an escaped U+FFFD before the text reaches System.Text.Json. This is the part that actually fixes the reported bug, because testhosts already in the wild emit these escapes and the runner has to cope with them. A vectorized IndexOf(""\\u"") keeps the cost off the common path, and nothing is allocated unless a replacement is needed.

Writer (Jsonite.cs). WriteString no longer emits an unpaired surrogate escape at all; complete pairs are still written as two escapes. Without this, a newly patched .NET Framework testhost would still break an older System.Text.Json runner under SDK version skew.

The affected character is now reported as U+FFFD instead of killing the run, which matches what the .NET testhost has always done and what the Newtonsoft-based runner effectively did.

Verified

Reproduced end to end with a net48 testhost throwing an exception whose message ends in a lone high surrogate: the run aborts on main at TestRequestSender.OnExecutionMessageReceived and reports 1 failed, 7 passed with this change.

[Copilot]

Copilot review overview

🔵 Needs a closer look

Cross-target JSON handling changes affect test-run reliability and warrant final human review.

Review effort: Lite
Findings: None

What changed in this PR

Fixes test-run aborts caused by unpaired UTF-16 surrogate escapes in JSON.

Changes:

• Repairs invalid surrogate escapes before System.Text.Json parsing.
• Sanitizes unpaired surrogates in Jsonite output.
• Adds cross-target regression tests.

File	Description
test/​Microsoft.TestPlatform.CommunicationUtilities.UnitTests/​JsonDataSerializerSurrogateRegressionTests.cs	Adds regression and edge-case coverage.
src/​Microsoft.TestPlatform.CommunicationUtilities/​Serialization/​JsonSurrogates.cs	Implements surrogate repair logic.
src/​Microsoft.TestPlatform.CommunicationUtilities/​JsonDataSerializer.Stj.cs	Applies repair to STJ parsing paths.
src/​Microsoft.TestPlatform.CommunicationUtilities/​Json/​Jsonite/​Jsonite.cs	Sanitizes unpaired surrogates when writing JSON.

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