fix: Secure token storage to prevent memory exposure (#12)

* fix: Secure token storage to prevent memory exposure

This commit addresses critical security vulnerabilities related to token
exposure in memory by implementing a hybrid approach that combines secure
storage with on-demand connection string building.

**Security Issues Fixed:**
- Token stored in cached connection strings (indefinite memory exposure)
- Plain std::string storage susceptible to memory dumps
- Tokens potentially visible in error messages and logs
- No secure zeroing of sensitive data after use

**Changes:**

1. **SecureString Implementation** (include/databricks/internal/secure_string.h)
   - Custom allocator with memory locking (mlock/VirtualLock)
   - Secure zeroing using volatile writes to prevent compiler optimization
   - Platform-specific implementations (Windows/Linux/macOS)
   - Utility functions for safe token handling

2. **AuthConfig Security Enhancement** (include/databricks/core/config.h)
   - Added private SecureString secure_token_ member
   - Added set_token() and get_secure_token() methods
   - Maintained backward compatibility with public token field
   - Updated config loading to use secure storage

3. **Connection String Caching Removed** (src/core/client.cpp)
   - Removed cached_connection_string member
   - Build connection strings on-demand only
   - Secure zero immediately after SQLDriverConnect() call
   - Clear ODBC output buffer that may contain sensitive data

4. **Token Sanitization** (src/core/client.cpp)
   - Added sanitize_error_message() to redact tokens from logs
   - Pattern matching to remove PWD=<value> from error strings
   - Applied to all error logging locations

5. **PoolKey Security** (src/internal/pool_manager.h/cpp)
   - Changed token storage from std::string to SecureString
   - Updated hash computation and equality operators

6. **Security Test Suite** (tests/unit/core/security_test.cpp)
   - 12 comprehensive test cases
   - Validates secure zeroing, token handling, and backward compatibility

**Backward Compatibility:**
- No breaking API changes
- Public token field still works
- Gradual migration path available
- Suitable for minor version bump (v0.2.5 or v0.3.0)

**Testing:**
- All 120 tests pass (108 existing + 12 new security tests)
- No regressions introduced
- Verified on macOS with AppleClang 17.0.0

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* Remove Basic Tokens

* Update Token References and Unit Tests

* Update Verbose README.md

* v0.3.0 Tag Update

---------

Co-authored-by: Claude <[email protected]>

Author: Calvinjmin

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.14)
 project(databricks_sdk
-    VERSION 0.2.4
+    VERSION 0.3.0
     DESCRIPTION "Databricks C++ SDK"
     LANGUAGES CXX)