fix(cache): avoid misleading message on refs not built "on purpose"

[marcelolynch]

cache get on a fork printed a "no cache found for HEAD" note whenever the per-SHA fork marker was absent, even when every file was then served from mathlib's master cache — which is always the case for a tooling-only PR (its CI build stages zero files, so the upload job and the marker are skipped by design). Reported in #41707 (comment).

The note is now printed after the download rounds and only when files are actually missing; cache query's no-result message names both possible causes (CI hasn't built the commits, or its builds had nothing fork-specific to upload) instead of claiming the first.

🤖 Generated with Claude Code

[github-actions]

PR summary a022a0f01d

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

+ DownloadResult
+ containsSub
+ forkHintSHA?
+ missingFilesForkLines
+ missingFilesGenericLines
+ test_forkHintSHA_guards
+ test_missingFilesLines
+ warnIfMissingFiles
- informIfHeadNotBuilt

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit a022a0f).

• +0 new declarations
• −0 removed declarations

No declaration differences.

---

No changes to strong technical debt.

No changes to weak technical debt.

Current commit a022a0f01d
Reference commit 3de5ed81cc

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).