feat: migrate to new flag engine with context value support

[gagantrivedi]

Summary

This PR upgrades the SDK to use the new flag engine with context value support and includes critical error handling improvements and validation enhancements.

Changes

🚀 Engine Upgrade

• Migrated to new flagsmith-flag-engine architecture in order to support context values
• Fixed get_environment_flags() to properly ignore segment overrides
• Updated evaluation context handling for better separation of environment vs identity evaluation

🐛 Critical Bug Fixes

• Fixed swapped error messages: FlagsmithClientError and FlagsmithAPIError were displaying incorrect messages
• Fixed background thread crashes: Replaced .unwrap() with proper error handling in polling thread to prevent silent failures
• Added graceful degradation: When environment updates fail, the client now logs warnings and falls back to API mode instead of crashing

✨ Enhancements

• Server key validation: Added validation to ensure local evaluation mode only accepts server-side keys (starting with ser.)
• std::error::Error trait: Implemented standard error trait for better ecosystem integration with anyhow, eyre, and other error handling crates
• Improved resilience: Background polling thread now retries on failures instead of dying silently

🧪 Tests

• Added integration tests for segment filtering in environment flags
• Added tests for identity segment membership
• Updated test fixtures with multivariate feature support

Breaking Changes

None - all changes are backward compatible

Related Issues

Fixes issues with:

• Silent background thread failures causing stale data
• Confusing error messages showing wrong error types

[Zaimwa9]

Overall looks good, couple of questions. Even though I'm positively surprise by the ease of reading/manipulating Rust, I might not be the best suited one for syntax/language specifics

[Zaimwa9]

Ok thanks for your answers. To me it's good like this 👍