s3: match the SymLink metadata key case-insensitively in ReadLink

[barnumbirr]

Description of the Change

S3 lowercases user metadata keys. SymLink writes "SymLink", but ReadLink
looks up output.Metadata["SymLink"] and the value comes back under
"symlink" — so the lookup misses and ReadLink returns an empty string with a
nil error. To the caller that reads as "this object is not a link" rather
than as a failure.

deb.packageIndexByHash uses ReadLink to find the physical file its .old
pointer refers to, and removes it before rotating:

linkTarget, err = file.parent.publishedStorage.ReadLink(oldIndexPath)
if err == nil {
    // If we managed to resolve the link target: delete it. This is the
    // oldest physical index file we no longer need.
    _ = file.parent.publishedStorage.Remove(linkTarget)
}

With "" returned, the Remove is a no-op and its error is discarded. The
pointers rotate correctly, so the bug is invisible in the published metadata —
but every superseded by-hash index file is left behind, one per index file per
hash algorithm per publish, indefinitely.

Filesystem backends are unaffected: they have real symlinks and never go
through this path.

Evidence

Measured on a live R2-backed archive publishing three suites a few times a day,
with Acquire-By-Hash enabled on 2026-08-24:

dists/ total 711  ->  by-hash 672, plain 39

Only 39 index files are current. The other 672 are orphaned by-hash copies,
accruing at roughly 48 per publish. Reading the metadata back from R2 directly
shows the casing:

Packages       metadata keys: ['symlink']
Packages.old   metadata keys: ['symlink']

Why the existing test does not catch it

TestSymLink round-trips SymLink → ReadLink, but ends with
c.Skip("copy not available in s3test"), so it never runs — the test server has
no CopyObject. This path has therefore never been covered.

The regression test added here writes the metadata with PutObject instead, so
it needs no CopyObject support. It fails before the change with
obtained string = "" and passes after.

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional) — the functional
  suite has no S3 backend; the unit test covers the decoding directly
• man page updated (if applicable) — no user-visible interface change
• go build ./s3/..., go vet ./s3/..., gofmt -l s3/ and go test ./s3/...
  all clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.08%. Comparing base (f59b0d2) to head (ed180e8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1625      +/-   ##
==========================================
- Coverage   77.37%   77.08%   -0.30%     
==========================================
  Files         165      165              
  Lines       15747    15750       +3     
==========================================
- Hits        12185    12141      -44     
- Misses       2356     2408      +52     
+ Partials     1206     1201       -5     

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