Extract PR size (shared)

Shared logic for the code-review Slack commands (send-code-review-slack, generate-code-review-slack-command) to derive the PR-size field. Kept in one place so the two commands can't drift.

How to extract

1. The 📏 Auto Label PR Size workflow (.github/workflows/pr-size-labeler.yml) applies exactly one size label to every PR. Read the PR labels and look for a size/* label: 🔵 size/XS, 🟢 size/S, 🟡 size/M, 🟠 size/L, or 🔴 size/XL.
2. Store the bucket as {emoji} {BUCKET} (e.g. 🟠 L) — just the bucket, no line counts.
3. Fallback (only if no size/* label is present yet — the workflow may not have run): derive the bucket from the PR's changed lines (additions + deletions), applying the same ignore list and thresholds as the workflow so the fallback can't land in a different bucket than the labeler. The workflow (.github/workflows/pr-size-labeler.yml) is the single source of truth for both:
   • Ignore list — read IGNORE_GLOBS in the workflow (lockfiles, generated REST clients under sdks/*/src/opik/rest_api/**, and snapshot/image files). Do not re-list the globs here; they would drift.
   • Thresholds — read BUCKETS in the workflow: XS < 20, S 20–100, M 101–300, L 301–600, XL > 600. Store the result as {emoji} {BUCKET}.
4. GitHub is the source of truth; the size at message-publish time is good enough (PRs rarely change bucket after review starts).

Message field

Include a size line in the message template, bucket only (no +/- counts):

:straight_ruler: pr size: {{pr_size}}