actor: add ErrMessageDropped for Ask path when mailbox drops

[erickcestari]

When the BackpressureMailbox's RED policy drops a message, Send returns false but neither context is cancelled. The Ask path previously completed the promise with fn.Err[R](ctx.Err()) where ctx.Err() is nil, producing a confusing nil-wrapped error. Add a three-way check that returns ErrMessageDropped in this case.

BackpressureMailbox will be added on: #10219

[gemini-code-assist]

Summary of Changes

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This pull request refines the error handling within the actor system's Ask mechanism. It introduces a dedicated error, ErrMessageDropped, to clearly signal when a message is rejected by a mailbox's backpressure policy. This change prevents confusing nil-wrapped errors in scenarios where a message is dropped but no context cancellation has occurred, thereby improving the clarity and predictability of error responses for Ask operations.

Highlights

• New Error Type: Introduced ErrMessageDropped to specifically indicate when a message is rejected by a mailbox's drop policy (e.g., RED backpressure) without context cancellation.
• Improved Error Handling in Ask Path: Modified the Ask method in actor/actor.go to return ErrMessageDropped when a message is dropped by the mailbox and neither the actor's context nor the Ask call's context is cancelled, resolving a previous issue where a nil-wrapped error was returned.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a bug in the Ask pattern where a message dropped by a mailbox's policy would result in a confusing nil-wrapped error. By introducing ErrMessageDropped and adding a three-way check, the new logic properly handles this scenario. My only suggestion is to add unit test coverage for this new error path to ensure its robustness, which might require a minor refactoring to allow injecting a mock mailbox for testing.

[gemini-code-assist]

This change introduces a new error path for when a message is dropped by the mailbox policy. However, there are no tests to cover this new logic. With the current ChannelMailbox, this path is unreachable.

To ensure the correctness of this logic, especially with the future introduction of BackpressureMailbox, I recommend adding a unit test. This might require a small refactoring to allow injecting a mock Mailbox during actor creation for testing purposes. For example, the ActorConfig could be extended to optionally take a Mailbox instance, which would make the actor system more flexible and easier to test.