docs: add Thrift protocol test suite design (#74)

Add comprehensive design document for ADBC driver Thrift protocol test
suite.

Key features:
- Language-agnostic test specifications (~300 test cases across 16
categories)
- Standalone Go proxy server for failure injection testing
- Multi-language support strategy (C#, Java, C++, Go)
- Extractable design for future common repository

Initial implementation targets C# ADBC driver with plans to extend to:
- Java (JDBC driver)
- C++ (ODBC driver)
- Go (ADBC driver)

The design enables comprehensive testing of:
- Session lifecycle and management
- Statement execution (sync/async)
- Metadata operations
- Arrow format and compression
- CloudFetch results and failure scenarios
- Parameterized queries
- Error handling and recovery
- Concurrency and edge cases

Directory structure:
- docs/designs/thrift-protocol-tests/ - Design and specifications
- test-infrastructure/proxy-server/ - Standalone proxy (to be
implemented)

Related to runtime Thrift tests and ADBC E2E tests, but focuses on
comprehensive protocol compliance and failure scenario testing.

## What's Changed

Please fill in a description of the changes here.

**This contains breaking changes.** <!-- Remove this line if there are
no breaking changes. -->

Closes #NNN.

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Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: eric-wang-1990