fix(cli): gc 'print'/'tag' progress no longer shows NaN% or misleading '0 assets'

[lemon0333]

Fixes #625

Problem

Running cdk gc --action print (and --action tag on a bucket/repo that was already tagged) produced output that made users believe garbage collection was broken, even though it was working:

[NaN%] 0 files scanned: 0 assets (0.00 MiB) tagged, 0 assets (0.00 MiB) deleted.
[100.00%] 13 files scanned: 0 assets (0.00 MiB) tagged, 0 assets (0.00 MiB) deleted.
[NaN%] 0 files scanned: 0 assets (0.00 MiB) tagged, 0 assets (0.00 MiB) deleted.

Two things are wrong:

1. [NaN%] — ProgressPrinter.print() computes assetsScanned / totalAssets. For an empty bucket/repo (or the final flush of one) totalAssets is 0, so 0 / 0 renders as NaN%.
2. "0 assets tagged, 0 assets deleted" — the print action intentionally never tags or deletes, so those counters are always 0. The message therefore reads as "nothing to do" even when stale/isolated assets exist and would be collected.

Fix

• Guard the percentage math so an environment with nothing to scan reports [100.00%] instead of [NaN%].
• For the print action, report the assets that are eligible for garbage collection (isolated assets not referenced by any deployed stack) — e.g. N assets (X MiB) eligible for deletion — instead of the misleading 0 tagged, 0 deleted line. The tag / delete-tagged / full output is unchanged.

This is a messaging/counter clarity fix only; garbage-collection semantics are unchanged.

Added unit tests under the gc test suite covering the NaN guard, a known-total percentage, and the print-action eligible-asset messaging.

Follow-up (not in this PR): a repeated tag run reports 0 tagged because the assets are already tagged; clarifying that "already tagged" state is a separate, larger change and left for a follow-up.

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.13%. Comparing base (7ff50e7) to head (840603f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1873      +/-   ##
==========================================
+ Coverage   91.10%   91.13%   +0.03%     
==========================================
  Files          80       80              
  Lines       12205    12205              
  Branches     1742     1744       +2     
==========================================
+ Hits        11119    11123       +4     
+ Misses       1050     1046       -4     
  Partials       36       36              

Flag	Coverage Δ
suite.unit	91.13% <ø> (+0.03%)	⬆️

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