Deselect VGF tests when model-converter is not installed (#19282)

Summary:

VGF tests require the Arm MLSDK model-converter binary which is not provisioned in CI when `_ENABLE_VGF = False` (the current default in targets.bzl). These tests correctly skip via `SkipIfNoModelConverter`, but TestInfra reports skipped/disabled tests as FAILURE in the OMH dashboard, creating ~1,025 phantom failures for the ai_infra_mobile_platform oncall.

This change uses the existing `pytest_collection_modifyitems` hook in conftest.py to deselect (not collect) tests that carry the `SkipIfNoModelConverter` marker (detected via `reason` string on `skipif` markers). This is more precise than name-based filtering because:
- It catches parametrized variants like `test_roundtrip[VGF]` that use `marks=SkipIfNoModelConverter`
- It does NOT deselect tests like `test_compile_spec_vgf_no_quant` that have "vgf" in the name but do not need model-converter

Deselected tests are invisible to TestInfra, eliminating false failure reports. When `_ENABLE_VGF` is later flipped to True and model-converter is provisioned, VGF tests will automatically be collected and run again.

Differential Revision: D103644035

Author: psiddh

backends/arm/test/conftest.py

@@ -29,7 +29,21 @@ def pytest_configure(config):
 
 
 def pytest_collection_modifyitems(config, items):
-    pass
+    from executorch.backends.arm.test.runner_utils import model_converter_installed
+
+    if not model_converter_installed():
+        deselected = []
+        remaining = []
+        for item in items:
+            for marker in item.iter_markers("skipif"):
+                if "model-converter" in marker.kwargs.get("reason", ""):
+                    deselected.append(item)
+                    break
+            else:
+                remaining.append(item)
+        if deselected:
+            config.hook.pytest_deselected(items=deselected)
+            items[:] = remaining
 
 
 def pytest_addoption(parser):