test: Resolve CI warnings and add comprehensive BingTextSearch LINQ filter tests

Fix compiler warnings blocking CI/CD builds:
- Fix CS8602 nullable warnings: Add null-forgiving operators (!) to 5 test locations
- Fix CS0618 obsolete warning: Add pragma to suppress ITextSearch deprecation warning during Phase 2 transition

Add 18 new comprehensive tests covering edge cases and complex scenarios:
- Boolean value handling (true/false/inequality with safeSearch parameter)
- Multiple Contains operations (3+ conditions with distinct operators)
- Mixed operators (equality + Contains + negation in single filter)
- Result structure preservation across all 3 search methods
- URL encoding validation for special characters
- Null filter handling and contradictory conditions
- All 6 supported BingWebPage properties (Name, Url, DisplayUrl, Snippet, Language, IsFamilyFriendly)

Test quality improvements:
- Fix 2 test assertions for correct Bing URL encoding expectations
- Document limitation: Query parameters don't support negation prefix
- Verify semantic correctness of LINQ→Bing query translation

All 56 BingTextSearch tests passing (100% success rate)
Builds clean with TreatWarningsAsErrors=true

Contributes to #10456 (LINQ filtering migration initiative)

Author: alzarei