Thread Safety & Sendable Conformance Fixes

[leogdion]

Parent Issue for Thread Safety and Testing Improvements

This parent issue groups together critical thread safety fixes, Sendable conformance updates, and related test improvements.

Child Issues:

• CRITICAL: DateFormatter thread safety issue in Date+RFC2822 #134 - CRITICAL: DateFormatter thread safety issue in Date+RFC2822
• Add Sendable conformance to FileManagerHandler #135 - Add Sendable conformance to FileManagerHandler
• Make DataSourceTests.testSendable properly async #136 - Make DataSourceTests.testSendable properly async
• Add test coverage for invalid RFC 2822 date formats #137 - Add test coverage for invalid RFC 2822 date formats

Scope:

1. Fix the critical DateFormatter thread safety issue that can cause race conditions
2. Add missing Sendable conformance to public types per coding guidelines
3. Fix async test implementations to properly await Tasks
4. Add comprehensive test coverage for RFC 2822 date parsing edge cases

Branch: fix/thread-safety-sendable

Priority: HIGH (Contains critical bug fix)