ci: rewrite README links to absolute URLs for PyPI (#3074)
* ci: rewrite README links to absolute URLs for PyPI
PyPI renders the README from the package metadata baked in at build time and ships no other repo files, so the README's relative links (docs/*, CONTRIBUTING.md) 404 on the project page.
In the release-build SDK job, rewrite relative Markdown links to absolute GitHub blob URLs pinned to the release tag, editing the checked-out README in place (never committed) before `uv build` bakes it into the long_description. A follow-up verify step fails the build if any relative link remains in the wheel METADATA or sdist PKG-INFO. Images and absolute links are left untouched.
Fixes #1375
* Improve error handling
