feat(layers/concurrent-limit): accept custom semaphore without API break

[PsiACE]

Which issue does this PR close?

Closes #6591 .

Port #6618 with fixes.

Rationale for this change

Add support for passing a shared/custom semaphore to ConcurrentLimitLayer without breaking existing APIs.

What changes are included in this PR?

• Introduce ConcurrentLimitSemaphore abstraction (layer-named) with default Arcmea::Semaphore implementation.
• Keep ConcurrentLimitLayer::new(permits) intact; add with_semaphore(...) for shared/custom semaphore; align HTTP setter naming.
• Add tests for shared/custom semaphore usage.

Are there any user-facing changes?

Yes. ConcurrentLimitLayer can now accept a custom/shared semaphore via with_semaphore(...) while keeping the existing new(permits) constructor.

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[Copilot]

Pull request overview

This PR adds support for custom/shared semaphores to ConcurrentLimitLayer without breaking the existing API. It introduces a new ConcurrentLimitSemaphore trait abstraction that allows users to provide their own semaphore implementations while maintaining backward compatibility with the existing new(permits) constructor.

Key changes:

• Introduced ConcurrentLimitSemaphore trait to abstract semaphore implementations
• Added with_semaphore() and with_http_semaphore() methods for custom semaphore injection
• Refactored internal types to be generic over the semaphore implementation
• Added tests demonstrating shared and custom semaphore usage

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 5 comments.

File	Description
core/layers/concurrent-limit/src/lib.rs	Core implementation: added ConcurrentLimitSemaphore trait, made layer/accessor/wrapper types generic over semaphore type, added new API methods, and included tests for shared and custom semaphores
core/layers/concurrent-limit/Cargo.toml	Added tokio as dev-dependency for async test support
core/Cargo.lock	Updated lock file to reflect new tokio dev-dependency

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[PsiACE]

The failed CI appears unrelated to this PR.