Fix partner-impact Slack notice crashing when a PR changes more than 20 partner files

[hua7450]

Summary

The partner-api-impact-notice.yaml workflow (Slack notice when partner contract tests change) crashes on any PR that changes more than 20 partner files — first triggered by #8890 (172 changed partner files), which consequently got no Slack notice.

Root cause

file_list=$(printf '%s\n' "${CHANGED_FILES}" | sed '/^$/d' | head -20 | sed 's/^/- /')

head -20 exits after emitting 20 lines, closing the pipe's read end. GNU sed flushes its output in ~4KB chunks, so with enough files its next write() hits the closed pipe. The Actions runner ignores SIGPIPE, so sed gets EPIPE, prints sed: couldn't write 114 items to stdout: Broken pipe, and exits 4 — which set -euo pipefail promotes to a job failure. Reproduced deterministically: a producer still writing after the consumer exits fails with 141/4 under pipefail; the awk form below cannot fail this way because awk always reads to EOF.

Fix

• Replace the head -20 pipeline with awk 'NF && ++n <= 20 { print "- " $0 }' — identical output (blank lines dropped, first 20 files, - prefix), but the consumer reads all input, so no producer ever receives SIGPIPE.
• Make the list heading accurate: Showing first 20: only when the list is actually truncated, Changed files: otherwise.

Message rendering dry-run against #8890's real 172-file list produces the expected notice (Changed partner files: 172, first 20 listed, ...and 152 more).

Note: this workflow runs on pull_request_target, so the fix only takes effect after merging to main; #8890's red check clears on its next event after that.

Test plan

• YAML parses (PyYAML)
• awk filter verified on blank-line and 172-line inputs
• Full fixed step dry-run rendered the exact Slack payload for Update API partner contract tests to match live partner traffic #8890
• CI passes

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