fix: install libpng-dev and pin reticulate to correct Python for CI

[kinyoklion]

Summary

The CI build was failing due to two issues:

1. Missing libpng-dev system package
The reticulate R package's dependency png fails to compile on ubuntu-latest because libpng-dev is not installed. Added an explicit step to install it.

/bin/bash: line 1: libpng-config: command not found
read.c:3:10: fatal error: png.h: No such file or directory
ERROR: compilation failed for package 'png'
ERROR: dependency 'png' is not available for package 'reticulate'

2. reticulate using wrong Python
After fixing the first issue, reticulate was downloading its own Python 3.12 via uv instead of using the Python 3.8 where launchdarkly-server-sdk was pip-installed, causing ModuleNotFoundError: No module named 'ldclient'. Fixed by creating a venv from the setup-python installation and setting RETICULATE_PYTHON to point to it.

Root cause run: failed run

Review & Testing Checklist for Human

• Verify CI passes on this PR (it did on the latest push, but confirm it stays green)
• Confirm the RETICULATE_PYTHON + venv approach is the preferred way to pin reticulate to a specific Python — an alternative would be calling reticulate::use_python() in main.r or setting RETICULATE_PYTHON_ENV
• Consider whether Python 3.8 (EOL) should be bumped to a supported version in a follow-up — this is a pre-existing concern, not introduced by this PR

Notes

• Requested by: @kinyoklion
• Devin Session

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