Summary: Refactor create_midstream_matcher Regex for Readability & Investigate Optional Upstream Tag Support

Description

We recently simplified the create_midstream_matcher function to strictly enforce the +rhai suffix. See https://gitlab.com/redhat/rhel-ai/wheels/builder/-/merge_requests/1708 . This ensures that non-midstream tags (formerly "upstream") are rejected, preventing build failures caused by processing tags that lack necessary dependencies.

While the logic is now safer, we need to improve the maintainability of the regular expressions and determine if we need to re-introduce upstream support behind a feature flag.

Context & Recent Changes

• Strict Enforcement: The conditional logic for upstream tag support was removed.

• Mandatory Suffix: The +rhai suffix is now required in all regex patterns.

• Terminology: Updated "upstream" to "non-midstream" throughout the codebase.

• Scope: Applied to midstream built packages (llama-stack, vLLM for now).

Objectives

1. Refactor Regex: Convert the current condensed regular expressions to use Verbose Syntax (e.g., re.VERBOSE in Python). This will allow us to comment on individual parts of the pattern for better long-term maintainability.
2. Optional Support (If required): If teams need upstream tags, implement an optional argument (llamastack will need it). A new argument will be added to the matcher, rejecting upstream tags by default, but allowing projects to building from upstream tags as well.