fix(configdb-tests): use Assert.SkipUnless + fast-fail connect timeout for MSSQL fixture (#23)

Reviewer of Bundle C (#23 M1) flagged two blockers in the
AddAuditLogTableMigration integration tests:

1. Tests used 'if (!await EnsureMigrationApplied()) return;' which made
   the xunit runner report them as Passed when the dev MSSQL container
   was absent — a CI false-positive risk. xunit 2.9.x does NOT ship the
   v3 Assert.Skip/SkipUnless/SkipWhen API surface (verified empirically
   against xunit.assert 2.9.3 — only v3.x exposes those static methods),
   so the canonical xunit-v2 equivalent is the Xunit.SkippableFact
   package. Replaced [Fact] with [SkippableFact] and the early-return
   pattern with 'Skip.IfNot(_fixture.Available, _fixture.SkipReason)' as
   the first statement of each of the 8 audit-log test methods. The
   runner now reports them as Skipped (not Passed) when MSSQL is down.

2. MsSqlMigrationFixture relied on SqlClient's 30s default connect
   timeout, so a no-container fixture construction hung ~30s. Added
   'Connect Timeout=3' to DefaultAdminConnectionString. Verified
   fail-fast under ~4s end-to-end with a bad host via env-var override.

Additional fixture cleanups:
- Migration is now applied once in the fixture constructor (was per-test
  via EnsureMigrationApplied for idempotency). Tests reach a fully-
  migrated database with no extra setup. Removed the now-unused
  EnsureMigrationApplied helper from the test class.
- Constructor narrowed its catch to SqlException + InvalidOperationException
  for the OpenAsync step (the only legitimate connect-failure surfaces);
  everything else (CREATE DATABASE, MigrateAsync) is treated as a hard
  fixture failure and bubbles up. Added a best-effort
  TryDropOrphanDatabase() pre-throw cleanup so partial construction
  cannot leak guid-suffixed databases.
- Stale doc comments referencing the (non-existent) xunit 2.9.x Skip
  shim removed; replaced with accurate notes about Xunit.SkippableFact.

Verified:
- dotnet build ScadaLink.slnx: clean (0 warnings, 0 errors).
- dotnet test ScadaLink.ConfigurationDatabase.Tests with MSSQL up:
  Passed 150 / Skipped 0 / Failed 0.
- Same suite with SCADALINK_MSSQL_TEST_CONN pointed at a closed port:
  the 8 AddAuditLogTableMigration tests report as Skipped (visible
  '[SKIP]' lines in runner output), total elapsed ~3s.

Files touched:
- Directory.Packages.props: added Xunit.SkippableFact 1.5.61.
- tests/ScadaLink.ConfigurationDatabase.Tests/ScadaLink.ConfigurationDatabase.Tests.csproj:
  added the SkippableFact PackageReference.
- tests/ScadaLink.ConfigurationDatabase.Tests/Migrations/MsSqlMigrationFixture.cs:
  Connect Timeout=3, constructor refactor, doc-comment fixes.
- tests/ScadaLink.ConfigurationDatabase.Tests/Migrations/AddAuditLogTableMigrationTests.cs:
  [SkippableFact] + Skip.IfNot pattern across all 8 tests.

Untouched (per reviewer guidance):
- Migration file (Bundle C main artifact unchanged).
- Bundle B reconciliation (composite PK + UX_AuditLog_EventId).
- SqlClient VersionOverride 6.1.1 in the test csproj.
- infra/* (separate uncommitted local edits remain in working tree).

Directory.Packages.props

@@ -64,6 +64,14 @@
     <PackageVersion Include="System.IdentityModel.Tokens.Jwt" Version="8.11.0" />
     <PackageVersion Include="xunit" Version="2.9.3" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="3.1.4" />
+    <!--
+      Xunit.SkippableFact provides [SkippableFact] + Skip.IfNot/Skip.If for
+      xunit v2. The native Skip API (Assert.Skip / Assert.SkipUnless /
+      Assert.SkipWhen) only exists in xunit v3; xunit 2.9.x lacks it. Used by
+      Bundle C MSSQL integration tests in ScadaLink.ConfigurationDatabase.Tests
+      to mark tests as Skipped (not silently Passed) when MSSQL is unreachable.
+    -->
+    <PackageVersion Include="Xunit.SkippableFact" Version="1.5.61" />
   </ItemGroup>
 
 </Project>