pdn: test a ring layer named by -pins

[shivarammysore]

Adds the case from #11209, which @gadfort confirmed is intended behaviour ("yes rings can become pins so that is correct") and offered to review.

core_grid_ring_on_pin_layer covers the rings_ half of the -pins validation in
Grid::checkSetup():

for (const auto& ring : rings_) {
  for (auto* layer : ring->getLayers()) {
    check_layers.insert(layer);
  }
}
for (const auto& strap : straps_) {
  check_layers.insert(strap->getLayer());
}

All fifteen existing cases that use -pins name a strap layer, so the straps_ loop alone
satisfies the check for every one of them — removing the rings_ loop leaves the pdn regression
green, and -pins <ring layer> would then raise PDN-0111 with nothing to catch it.

The case is core_grid_with_rings_with_straps with one option added, -pins metal6, the upper
layer of its ring. ⟹ its control is already in the suite: the existing case is the same design
without the option, so the two goldens differ only where the option acts — two block terminals,
two ports each (the ring's two metal6 sides), and unchanged SPECIALNETS geometry.

It reuses the design and LEF the existing case already reads, so it adds no new library data.

Registered in both src/pdn/test/CMakeLists.txt and src/pdn/test/BUILD.

The .defok and .ok were generated at 945a9f48dc6e5cc91d865daa92c45a1094cb682c (master at the
time of writing) and the case reports No differences found. there. Happy to regenerate if CI
disagrees.

On -max_rows/-max_columns: understood, leaving that for the CPP CI suite.

[gemini-code-assist]

Code Review

This pull request introduces a new integration test case, core_grid_ring_on_pin_layer, to verify that a layer occupied only by a ring is a valid -pins layer and that the ring's segments correctly become block pins. The test is added to both the Bazel (BUILD) and CMake (CMakeLists.txt) build configurations, along with its expected output (.ok) and TCL script (.tcl). No review comments were provided, so there is no feedback to address.

[gadfort]

LGTM