chore: unified hybrid tests enabled environment var

[NoelStephensUnity]

Purpose of this PR

---

Base branch: chore/enable-unified-hybrid-tests (#4122) — not develop-3.x.x — merge #4122 before this.

---

Follow-up to #4122, which added the unified (NGO + N4E) hybrid prefab test job. That first step
selected which tests ran by passing a name pattern to UTR (unified_test_filter). This PR replaces
that pattern with test selection defined in C#, so the YAML no longer decides what runs.

This is the first part of 3 PRs:

1. chore: enable unified hybrid tests #4122 gets the initial scaffolding into place (under review).
2. chore: unified hybrid tests enabled environment var #4137 (this PR) removes the test filter and uses the UNIFIED_TEST environment variable (almost exactly like CMB service tests with the difference that it is going to start out as a "turned off by default" for all tests and will be an "opt-in". The UniversalNetworkTransformTest is the only opt-in for 4137 ). 4137 also removes a bunch of the YAML comment verbosity along with other script file comments.
3. Yet to be posted PR that will include more NetworkTransformTests.

What replaces the test filter from 4122. The job now sets UNIFIED_TESTS: "true" and passes no --testfilter
(unified_test_filter is deleted from project.metafile). NetcodeIntegrationTest applies three
rules, all under #if UNIFIED_NETCODE, so nothing changes when N4E is absent:

Test pass	Test case	Result
non-unified	hybrid prefab	ignored — IgnoredWithoutUnifiedTestsReason
unified	hybrid prefab, not opted in	ignored — NotOptedInForUnifiedTestsReason
unified	not a hybrid prefab	ignored — IgnoredForUnifiedTestsReason
unified	hybrid prefab, opted in	runs

Supporting pieces:

• ApplyUnifiedTestFilter(HostOrServer) for tests that take HostOrServer as a test method
  parameter rather than a fixture argument. OneTimeSetup runs once per fixture and cannot see a
  method parameter, so those fixtures call this themselves before starting any instances.
  NetworkVariableTests is the case in this PR.
• IgnoreIfUnifiedTestsEnvironmentVariableSetAttribute plus
  NetcodeIntegrationTestHelpers.IgnoreIfUnifiedTestsEnvironmentVariableSet() for fixtures that do
  not derive from NetcodeIntegrationTest. The attribute respects RunState.NotRunnable first so it
  cannot resurrect a test another attribute already excluded.
• USE_CMB_SERVICE always wins. Distributed authority and hybrid prefab spawning cannot be the
  same run, so a CMB service pass forces unified off.
• The environment variable is read once per test set and reset in OnOneTimeTearDown.
  GetUnifiedTestsEnvironmentVariable() also honours a UNIFIED_TESTS compile define, which is what
  makes a local run possible without setting the variable in the editor's environment.
• Fixtures that do not need to run against hybrid prefabs now opt out explicitly, each with a comment
  saying to remove the opt-out if the test is later updated to support them.

Also in here: two missing $ prefixes on interpolated Log.Error strings in
NetworkManager.UnifiedIsConfiguredCorrectly, which were logging a literal {nameof(NetCodeConfig)}.

Jira ticket

MTT-XXXX

Changelog

Not relevant — CI and test-infrastructure only. The one runtime edit is an error-message fix in
unified code that is not yet publicly released.

Documentation

• No documentation changes or additions were necessary.

The reasoning that used to live in the project.metafile comment block now lives with the code it
governs: the UseUnifiedTests and ApplyUnifiedTestFilter XML docs explain why hybrid prefab cases
are opt-in and why a method-parameter HostOrServer needs separate handling.

Testing & QA (How your changes can be verified during release Playtest)

Functional Testing

Manual tests:

• Manual testing done — runtime tests rebuilt and run locally; the suite is green.

Automated tests:

• Covered by existing automated tests

The change is itself test infrastructure, so the verification is the shape of the run rather than a
new test: in a unified pass, UnifiedNetworkTransformTest runs and everything else reports as
ignored with one of the three reasons above; in a normal pass, nothing is ignored and the suite
behaves exactly as it did before this PR. Both are visible in the UTR results XML.

Does the change require QA team to:

• Review automated tests?
• Execute manual tests?
• Provide feedback about the PR?

Up-port

Not needed. This targets the #4122 branch on the develop-3.x.x line, and the unified API exists
only there.

Backports

Not needed, same reason.

[codecov-github-com]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@                Coverage Diff                @@
##           develop-3.x.x    #4137      +/-   ##
=================================================
- Coverage          78.01%   77.92%   -0.10%     
=================================================
  Files                153      153              
  Lines              26254    26260       +6     
=================================================
- Hits               20483    20463      -20     
- Misses              5771     5797      +26     

Flag	Coverage Δ
NGOv2_project_testproject_ubuntu_pinnedTrunk	77.64% <ø> (?)
NGOv2_project_testproject_ubuntu_trunk	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...netcode.gameobjects/Runtime/Core/NetworkManager.cs	79.78% <ø> (ø)
....netcode.gameobjects/Runtime/Core/NetworkObject.cs	75.29% <ø> (ø)

... and 4 files with indirect coverage changes

Components	Coverage Δ
com.unity.netcode.gameobjects	77.92% <100.00%> (-0.10%)	⬇️

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