pdn: various fixes to follow pin handling

[gadfort]

Summary

1. Account for MY rows orientations
2. ensure the smaller row pitch is selected, not just the first
3. use row height to add followpins to better account for non-1x row heights
   Fixes the incorrect double VSS straps in the power switch tests.

Type of Change

• Bug fix

Impact

Correct followpin ordering.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass.
• My code follows the repository's formatting guidelines.
• I have included tests to prevent regressions.
• I have signed my commits (DCO).

Related Issues

#10955

[gemini-code-assist]

Code Review

This pull request updates the PDN generation logic to support mirrored and multi-height standard cell rows. It introduces a method to determine the row pitch based on the minimum row height and refactors followpin shape generation to iterate over the row height. Feedback on the changes highlights two critical issues: a potential null pointer dereference if a row lacks a valid site in determinePitch, and a potential infinite loop in makeShapes if row_height_ is zero.