Problem

--list-tests ignored --treenode-filter and listed every test in the assembly. Users expect app --list-tests --treenode-filter X to list only the tests filter X selects — the same set a real run would execute.

Root cause

MTP routes --list-tests to a DiscoverTestExecutionRequest, which flows through TestDiscoveryService.DiscoverTests with isForExecution: false. The filter was applied only when isForExecution was true:

var filteredTests = isForExecution ? _testFilterService.FilterTests(filter, allTests) : allTests;

isForExecution legitimately gates side-effecting work — argument registration / fixture creation (#6151) and dependency-closure expansion. But filtering itself is pure (MatchesTest only compares the test's path/properties against the filter), so running it during discovery was always safe; it just was never wired up. Filtering got conflated with the side-effect gate.

Fix

Apply the filter during discovery too, but only when a real filter is present:

var hasRealFilter = filter is not (null or NopFilter);
var filteredTests = (isForExecution || hasRealFilter)
    ? _testFilterService.FilterTests(filter, allTests)
    : allTests;

• No filter (bare --list-tests, IDE Test Explorer) → null/NopFilter → returns all tests unchanged, including [Explicit] ones (which FilterTests would otherwise strip). No behavior change.
• Filter present → reuse FilterTests, so the listing mirrors exactly what a run with that filter would execute.
• No fixture leaks — RegisterTestsAsync is still called with isForExecution: false, so RegisterTestArgumentsAsync stays skipped.
• No dependency pull-in — the dependency-closure block stays guarded by isForExecution.

Engine-runtime only; works identically in reflection and source-generated modes since both flow through TestDiscoveryService. No snapshot/source-gen output changes.

Tests

Adds ListTestsFilterTests (reflection + source-generated modes):

• filter lists only matching tests,
• bare --list-tests still lists everything (guards the no-filter path).

Manually verified against TUnit.TestProject: