Fix $filter on searchable string fields (#27)

[paulirwin]

Fixes #27.

Problem

When an Edm.String field was Searchable=true (the default) and also Filterable=true, $filter queries like Category eq 'Electronics' returned zero results. The field was indexed only as a Lucene TextField, so the StandardAnalyzer lowercased and tokenized the value at index time ("Electronics" → "electronics"), but the filter path built an exact TermQuery from the raw literal and never matched. The reporter's workaround — setting IsSearchable=false — worked only because it collapsed the field into the StringField branch.

Per Filters in Azure AI Search and Analyzers for text processing: filters run on raw, non-analyzed content, independent of whether the field is searchable.

Fix

• SearchFieldExtensions.CreateField → CreateFields (now returns IEnumerable<IIndexableField>). For Edm.String fields that are searchable and filterable/sortable/facetable, index both a TextField (analyzed, for $search) and a StringField (raw, for $filter/$orderby/facets) under the same field name. Lucene merges postings by field name, so TermQuery matches the raw side while analyzed-token search still works.
• UpsertIndexDocumentActionBase.MergeDocument: remove existing fields once per distinct name before re-adding, so dual-indexed fields aren't dropped during upsert.
• ODataQueryVisitor: added EnsureFilterable(path) at the filter entry points — a filter referencing a field with Filterable=false now throws, matching Azure's behavior.

Test plan

• Unit: 108/108 pass, including three new tests in LuceneNetIndexSearcher_SearchableFilterableTests covering (a) Searchable=true, Filterable=true, (b) schema defaults (exact repro of Searchable and Filterable fields return no results with filters applied #27), (c) rejection on Filterable=false.
• Integration: 17/17 pass, including three new SDK/HTTP-level tests: issue Searchable and Filterable fields return no results with filters applied #27 repro, $search still matches tokenized terms on the dual-indexed field, and $filter on a non-filterable field errors.

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