Add plan document for async business rules priority execution

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Description

Async rules currently fire in parallel regardless of priority, defeating short-circuiting behavior. A priority-0 async rule failure doesn't prevent priority-1 async rules from starting since they've already been dispatched.

This PR adds a plan document (docs/dev/Async-Rule-Priority-Execution-Plan.md) proposing:

• New AsyncRuleExecutionMode enum with Parallel (default, backward-compatible) and Serial options
• New AsyncExecutionMode property on IBusinessRuleBase interface
• Priority-group-based execution algorithm in RunRules that:
  • Groups rules by priority
  • Executes serial async rules with await, enabling short-circuit checks
  • Fires parallel async rules without waiting (current behavior)

public enum AsyncRuleExecutionMode
{
    Parallel = 0,  // Current behavior, default
    Serial = 1     // Await completion, enable short-circuiting
}

The plan includes:

• Current implementation analysis
• Conceptual algorithm pseudocode
• Breaking changes assessment
• 5-phase implementation roadmap (10-17 days estimate)
• Open design questions for review

Original prompt

Create a plan in a markdown file for changing the way business rules are run in CSLA.rules.businessrules so async rules also honor priority alongside sync rules.

Async rules should get a property indicating that they can run serial or in parallel. Default to parallel like today.

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💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[StefanOssendorf]

How is the "grouping" inteded to be?
Assume the following Setup (all rules async):
R1, R2, R3 : Prio 1
R4, R5: Prio 2

Will R1, R2 and R3 run in parallel or serial before proceeding.

[mtavares628]

I think this solution will work well, and solve a lot of issues that I'm currently having with dependent async rules. How soon could this change be implemented?

[mtavares628]

I like this approach better, since it allows for sync and async rules at the same priority to run in the intended order instead of processing sync rules first and then async rules.

[mtavares628]

I would go with Option A to maintain consistency and give full control.

[mtavares628]

I also vote for option B here.

[mtavares628]

I like the idea of it having a timeout similar to CheclRulesAsync.

[mtavares628]

I think ProcessThroughPriority should affect serial async rules for consistency.

[mtavares628]

By making this async, it will also have the added benefit of awaiting for RunRules to finish before cycling through and running rules on affected properties. Currently, affected property rules are potentially being executed before the primary async rule has finished executing and as a result the affected property rules are using stale values from other properties that haven't been outputted yet from the original rule.