fix: restore PHP ksr generation test activation, update missing_feature comments

PHP implements ksr but doesn't support branch overrides in system-tests CI.
Keep the original test activated (it passes against released PHP once merged),
and mark new edge case tests as missing_feature due to branch override limitation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bm1549

manifests/php.yml

@@ -799,10 +799,9 @@ manifest:
   : '>=1.16.0'
   ? tests/parametric/test_sampling_span_tags.py::Test_Knuth_Sample_Rate::test_sampling_extract_knuth_sample_rate_distributed_tracing_tracecontext
   : '>=1.16.0'
-  tests/parametric/test_sampling_span_tags.py::Test_Knuth_Sample_Rate::test_sampling_knuth_sample_rate_not_set_for_default: missing_feature (ksr generation not implemented)
+  tests/parametric/test_sampling_span_tags.py::Test_Knuth_Sample_Rate::test_sampling_knuth_sample_rate_not_set_for_default: missing_feature (branch override not supported for PHP)
   ? tests/parametric/test_sampling_span_tags.py::Test_Knuth_Sample_Rate::test_sampling_knuth_sample_rate_overwritten_by_local_sampling
-  : missing_feature (ksr generation not implemented)
-  tests/parametric/test_sampling_span_tags.py::Test_Knuth_Sample_Rate::test_sampling_knuth_sample_rate_trace_sampling_rule: missing_feature (ksr generation not implemented)
+  : missing_feature (branch override not supported for PHP)
   tests/parametric/test_sampling_span_tags.py::Test_Sampling_Span_Tags::test_tags_appsec_enabled_sst011: bug (APMAPI-737)
   tests/parametric/test_sampling_span_tags.py::Test_Sampling_Span_Tags::test_tags_child_dropped_sst001: bug (APMAPI-737)
   tests/parametric/test_sampling_span_tags.py::Test_Sampling_Span_Tags::test_tags_child_kept_sst007: bug (APMAPI-737)