Implement HandlerResult trait to eliminate unnecessary trailing Flow::Continue

[konard]

Summary

This PR implements the HandlerResult trait as suggested in issue #4, eliminating the need for unnecessary trailing Flow::Continue returns in handlers.

Problem

Currently, every handler must return Try<Output = ()>, which requires explicit Flow::Continue returns even when handlers just perform some action and want to continue:

// Before: Unnecessary boilerplate :(
links.each(|link| {
    worker.work(link);
    Flow::Continue // <- Annoying!
});

Solution

The new HandlerResult trait allows handlers to return either () (implying Flow::Continue) or explicit Flow/Try types:

// After: No boilerplate needed! :)
links.each(|link| {
    worker.work(link);
    // Implicit continue - no trailing Flow::Continue needed!
});

Implementation Details

• HandlerResult trait: Converts various return types to Try<Output = ()>

  • () → Flow::Continue (the main benefit)
  • Flow → Flow (backward compatibility)
  • Any Try<Output = ()> → itself (extensibility)

• Updated trait methods: All each_*, create_*, update_*, and delete_* methods now accept HandlerResult instead of Try<Output = ()>

• Backward compatibility: Existing code with explicit Flow returns continues to work

Benefits

✅ Eliminates boilerplate: No more unnecessary Flow::Continue returns
✅ Backward compatible: Existing handlers with explicit Flow returns still work
✅ Type-safe: Compile-time guarantees about handler return types
✅ Flexible: Supports mixed usage patterns in the same codebase

Test Plan

• Created comprehensive examples demonstrating the new API
• Verified both () and explicit Flow returns work correctly
• Confirmed backward compatibility with existing patterns

Example Usage

The example in examples/handler_result_demo.rs demonstrates the improvement:

// New simplified API - the main benefit!
links.each(|link| {
    worker.work(link);
    // No trailing Flow::Continue needed!
});

// Explicit control flow still works when needed
links.each(|link| {
    process(link);
    if should_stop(link) {
        Flow::Break
    } else {
        // Can use () instead of Flow::Continue
    }
});

Fixes #4

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