fix: don't crash when /admin/serverinfo omits systemInfo

[phillipfickl]

Since Keycloak 26.4, GET /admin/serverinfo returns 200 but omits systemInfo for anyone who is not a master realm administrator (26.7 tightened this further to require manage-realm). fetchVersion() dereferenced the absent systemInfo unconditionally, so every resource accessor raised "Error: Call to a member function getVersion() on null" from library internals. A response carrying systemInfo without a version hit a second path, a TypeError out of SystemInfo::getVersion(), which is declared ': string' over a nullable property.

The version is only consumed by AttributeNormalizer::normalize() to strip outgoing properties gated by #[Since], the highest of which is major 26, and there are no #[Until] attributes at all. On every server affected by this bug the filter is therefore a guaranteed no-op: the library was making a privileged request on every accessor to compute nothing.

So version detection now degrades instead of throwing. A null version already means "no filtering", which on 26.4+ is identical to a successful detection. Detection is memoised via a separate flag so a failure is not retried on every subsequent accessor call, and the caught throwable is retained so the real cause survives.

getVersion() is the one explicit ask that cannot be answered without it, so that alone throws the new VersionDetectionException, naming both cause and remedy and chaining the original failure. This also repairs its return of a ?string from a ': string' signature.

Builder::withVersion() pins the version and skips the request. It is an optimisation, not the fix: with no #[Until] attributes, over-stating the version can never strip a field, so pinning is the only option that can be actively wrong when it drifts from the server.

Also rebuilds queryExecutor alongside serializer and commandExecutor, which otherwise kept a null-version serializer for the object's whole lifetime.

closes #184

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.24%. Comparing base (1c29894) to head (1874835).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
+ Coverage   92.18%   93.24%   +1.06%     
==========================================
  Files          32       32              
  Lines         972      977       +5     
==========================================
+ Hits          896      911      +15     
+ Misses         76       66      -10     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.