fix(adminui): never slice a DB-sourced string with the range operator (#504)

`Scripts.razor` rendered a hash prefix as `@s.SourceHash[..12]…`. `SourceHash`
is a plain nvarchar(64) with no length floor, so any value shorter than 12
characters threw ArgumentOutOfRangeException out of BuildRenderTree — unhandled
in Blazor, which takes the WHOLE page to an HTTP 500, not just the offending
row. Live-reproduced on docker-dev, where two Script rows carry `h1` and
`h-abs-hr200`: /scripts was a hard 500.

New `Components/Shared/DisplayText.Abbreviate(string?, int)`:
- null/blank renders the Admin UI's em-dash placeholder rather than throwing;
- a value already within budget renders verbatim with NO ellipsis, so the
  display never implies text that isn't there (the old markup appended "…"
  unconditionally, outside the slice — `h1` would have shown as `h1…`);
- a maxLength < 1 is clamped, so a display bug cannot become a page crash.

Applied to every unguarded range-operator slice over a DB-sourced string found
by sweeping the AdminUI for `[..N]`:

  Scripts.razor                    SourceHash    (the reported crash)
  Certificates.razor               Thumbprint    (read off the on-disk store)
  Deployments.razor  x2            RevisionHash
  Clusters/ClusterOverview.razor   RevisionHash

The other `[..N]` hits are safe and untouched: Guid.ToString("N") slices
(always 32 chars) and the `Length > 60 ? [..60]` ternaries that self-guard.

NOTE — the issue text blamed the docker-dev seed; that was wrong and is
corrected on #504. `seed-clusters.sql` inserts only ServerCluster, ClusterNode
and LdapGroupRoleMapping. The short-hash rows came from earlier live-gate
authoring. A freshly seeded rig is fine; the trigger is any Script row that did
not go through ScriptEdit / UnsTreeService.HashSource — REST-API authored,
hand-inserted, or migrated in.

Tests: 14 new in DisplayTextTests, covering the short/null/blank/clamped cases
and a never-throws sweep over every value x budget combination. AdminUI suite
739/739.

Live-verified on docker-dev (AdminUI has no bUnit): /scripts 500 -> 200 across
both central nodes, rendering `hash=h1` and `hash=h-abs-hr200` in full;
/deployments still truncates 64-char hashes at 12 with the ellipsis;
/certificates and /clusters/MAIN unchanged.

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Clusters/ClusterOverview.razor

@@ -58,7 +58,7 @@ else
             }
             else
             {
-                <div class="kv"><span class="k">Revision</span><span class="v mono">@_lastDeployment.RevisionHash[..16]…</span></div>
+                <div class="kv"><span class="k">Revision</span><span class="v mono">@DisplayText.Abbreviate(_lastDeployment.RevisionHash, 16)</span></div>
                 <div class="kv"><span class="k">Status</span><span class="v">@_lastDeployment.Status</span></div>
                 <div class="kv"><span class="k">Created</span><span class="v">@_lastDeployment.CreatedAtUtc.ToString("u")</span></div>
                 @if (_lastDeployment.SealedAtUtc is not null)