Add Snowflake database connection support

[Copilot]

This PR implements comprehensive Snowflake database support for DBChatPro, enabling users to connect to Snowflake data warehouses and leverage AI for intelligent query generation.

What's Added

Core Implementation:

• Created SnowflakeDatabaseService implementing the IDatabaseService interface following the established pattern used for MySQL, PostgreSQL, SQL Server, and Oracle
• Added official Snowflake.Data v4.1.0 NuGet package for reliable connectivity
• Implemented schema discovery using Snowflake's INFORMATION_SCHEMA.COLUMNS views
• Added robust SQL query execution with proper error handling for data type conversions

User Interface Integration:

• Added "SNOWFLAKE" option to the database type dropdown in the connection form
• Users can now select Snowflake alongside other supported database types
• Maintains consistent user experience with existing database connections

System Integration:

• Updated DatabaseManagerService to route Snowflake connections appropriately
• Registered SnowflakeDatabaseService in the dependency injection container
• Added sample connection string documentation for user guidance

Connection String Format

account=<your-account>;user=<your-username>;password=<your-password>;db=<your-database>;schema=<your-schema>;warehouse=<your-warehouse>;

Technical Details

The implementation follows the exact same architectural patterns as existing database services:

• Schema Discovery: Queries system tables to build table/column metadata for AI understanding
• Query Execution: Executes AI-generated SQL queries and returns results in standardized format
• Error Handling: Graceful handling of connection issues and data type conversion errors
• Dependency Injection: Proper service registration and lifecycle management

This enables users to connect their Snowflake data warehouses to DBChatPro and use AI to generate intelligent SQL queries based on their schema, making enterprise data analytics more accessible through natural language interaction.

Fixes #5.

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