fix(taxonomy): route Find<T>() to entry-filter endpoint when a taxonomy filter is set

Taxonomy.Find<T>() is `new`-hidden over Query.Find<T>() to support listing all
taxonomies (GET /taxonomies). Because C# resolves `new`-hidden members by the
static declared type, this silently broke the entry-filter feature
(Taxonomies().Above/Below/EqualAndAbove/EqualAndBelow/Exists(...).Find<Entry>()):
every call landed on GET /taxonomies instead of GET /taxonomies/entries, with
the filter dropped and no exception thrown - regardless of whether the filter
methods were chained fluently or called as separate statements.

Find<T>() now checks a new internal HasEntryFilters flag (true once a filter
method has populated QueryValueJson) and delegates to base.Find<T>() - the
inherited Query pipeline - when set, reusing the existing virtual _Url
dispatch to reach /taxonomies/entries with the filter intact.

Adds HasEntryFilters regression tests to TaxonomyUnitTests.cs, and tightens
TaxonomySupportTest.cs's assertions, which previously treated any exception
as a passing test and never verified the response shape.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: OMpawar-21