.Net: docs: Add ADR-0065 for LINQ-based ITextSearch filtering migration strategy

[alzarei]

Add ADR-0065: LINQ-Based Filtering for ITextSearch Interface

Summary

This PR adds a comprehensive Architectural Decision Record (ADR) documenting the decision to migrate ITextSearch from clause-based filtering to LINQ-based filtering using a dual interface pattern.

Context

Issue: #10456

The existing ITextSearch interface uses TextSearchFilter (clause-based approach) which:

• Creates runtime errors from property name typos
• Lacks compile-time type safety
• Requires conversion to obsolete VectorSearchFilter APIs internally
• Provides no IntelliSense support

Decision

Chosen Approach: Dual Interface Pattern (Option 3)

• NEW: ITextSearch<TRecord> with LINQ filtering (Expression<Func<TRecord, bool>>)
• LEGACY: ITextSearch marked [Obsolete] for backward compatibility
• Both interfaces coexist temporarily during migration

Key Architectural Points

Migration Strategy (Temporary by Design)

1. Phase 1 (Current): Both interfaces coexist - zero breaking changes
2. Phase 2 (Future Major Version): Increase deprecation warnings
3. Phase 3 (Future Major Version): Remove obsolete interface entirely

Why Mark as [Obsolete] Now

• Prevents new code from adopting legacy patterns
• Signals future removal to ecosystem
• Gives 1-2 major versions for migration
• Follows .NET best practices for API evolution

Implementation Patterns Documented

Pattern A: Direct LINQ Passthrough (VectorStoreTextSearch)

• Two independent code paths
• No conversion overhead
• Native LINQ support in vector stores

Pattern B: LINQ-to-Legacy Conversion (Bing, Google, Tavily, Brave)

• Converts LINQ expressions to legacy format
• Reuses existing API implementations
• Expression tree analysis

Benefits

✅ Zero breaking changes - existing code continues working
✅ Type safety - compile-time validation and IntelliSense
✅ AOT compatible - no [RequiresDynamicCode] attributes
✅ Clear migration path - deprecation warnings guide users
✅ Future-ready - establishes path for technical debt removal

Related PRs

This ADR documents the architectural decision behind:

• ✅ PR .Net: feat: Implement type-safe LINQ filtering for ITextSearch interface (microsoft#10456) #13175 - Core interfaces (Merged)
• ✅ PR .NET: Add ITextSearch<TRecord> with LINQ filtering and deprecate legacy ITextSearch (#10456) #13179 - VectorStoreTextSearch (Merged)
• ✅ PR .Net Improve type safety: Return TRecord instead of object in ITextSearch.GetSearchResultsAsync #13318 - Type improvements (Merged)
• ✅ PR .Net: feat: Modernize BingTextSearch connector with ITextSearch<TRecord> interface (microsoft#10456) #13188 - BingTextSearch (Merged)
• ✅ PR .Net: feat: Modernize GoogleTextSearch connector with ITextSearch<TRecord> interface (microsoft#10456) #13190 - GoogleTextSearch (Merged)
• ⏳ PR .Net: Net: feat: Modernize TavilyTextSearch and BraveTextSearch connectors with ITextSearch<TRecord> interface (microsoft#10456) #13191 - Tavily/Brave (Open)
• 📋 PR .Net: feat: Modernize samples and documentation for ITextSearch<TRecord> interface (microsoft#10456) #13194 - Samples/docs (Planned)

Review Notes

Per ADR process requirements:

• Status: accepted (implementation already complete)
• Deciders: @roji, @westey-m, @markwallace-microsoft (required reviewers)
• Consulted: @moonbox3
• Date: 2025-10-25 (when decision was finalized)

This ADR serves as:

• Permanent documentation of the architectural decision
• Reference for future maintainers
• Explanation of the temporary dual interface state
• Migration guide for ecosystem consumers

Checklist

• ADR follows template structure
• All required sections included
• Deciders listed as required reviewers
• Status set to accepted
• Date reflects decision finalization
• File naming follows convention: 0065-linq-based-text-search-filtering.md
• Located in correct directory: dotnet/docs/decisions/

Target Branch

• Base: feature-text-search-linq
• Reason: ADR documents the architectural decisions implemented in this feature branch