Sync vendored dotnet test wire contract from testfx (Expected/Actual field ids)

[Evangelink]

What

Reconciles the vendored dotnet test ↔ Microsoft.Testing.Platform wire contract with microsoft/testfx main.

The scheduled vendored-files drift check reported exactly one drifted source: src/Cli/dotnet/Commands/Test/MTP/IPC/ObjectFieldIds.cs (tracked against testfx's ServerMode/DotnetTest/IPC/ObjectFieldIds.cs). Upstream added two optional assertion-diff fields on FailedTestResultMessageFieldsId:

public const ushort Expected = 10;
public const ushort Actual = 11;

These carry the structured expected/actual values captured by assertion libraries so the terminal reporter can render an expected-vs-actual diff for multi-assembly runs. This PR ports those two field-id reservations to keep the named-pipe contract byte-aligned with testfx, and bumps the entry's baseline_ref_sha/baseline_blob_sha in eng/vendored-files.json so the drift workflow goes quiet.

The additions are appended after existing ids, and the serializer skips unrecognized field ids, so this is fully backwards compatible.

Deliberately not ported

• The upstream local-adaptation differences we always diverge on (file header, namespace, [Embedded] attributes).
• Upstream serializer ids 13 (WaitForServerControlRequest) and 14 (ServerControlMessage) — these belong to the reverse server-control pipe (protocol 1.4.0) and remain intentionally out of scope, as already noted in the local file.

Verification

python .github/scripts/check_vendored_files.py validate → Manifest OK: 30 entries, 38 sources.
python .github/scripts/check_vendored_files.py check --dry-run → Summary: 0 drifted, 0 errors.

Note on the MSTest/MTP version bump

The other half of the original request — bumping MSTest/MTP to "latest 3.4/2.4" — was not actioned here because it could not be resolved unambiguously: MSTest 3.4.3 (latest 3.4.x) is already present on the release/9.0.x branches, no Microsoft.Testing.Platform 2.4.x is published (latest public is 2.3.1), and these versions normally flow into the SDK automatically via darc/Maestro from the VMR rather than via a hand-authored PR. Happy to follow up once the target branch/exact versions are confirmed.

[Copilot]

Pull request overview

This PR syncs the vendored dotnet test ↔ Microsoft.Testing.Platform IPC wire-contract field IDs with the current microsoft/testfx main branch, ensuring the named-pipe protocol remains byte-aligned and compatible.

Changes:

• Added two new optional field IDs (Expected, Actual) to FailedTestResultMessageFieldsId to reserve upstream protocol slots for structured assertion diffs.
• Updated the corresponding eng/vendored-files.json baseline SHAs so the vendored-files drift check reflects the new upstream baseline.

Show a summary per file

File	Description
src/Cli/dotnet/Commands/Test/MTP/IPC/ObjectFieldIds.cs	Adds Expected/Actual field-id reservations (10/11) to keep the MTP IPC contract aligned with testfx.
eng/vendored-files.json	Bumps vendored baseline SHAs for the tracked upstream ObjectFieldIds.cs.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0