fix(limiter): release permit when inner factory rejects the call

[costela]

Both ConcurrencyLimiter variants acquire a permit before delegating inward, so an error from the inner factory (e.g. ErrCircuitOpen) has to release it again — no Observer is returned in that case, so nothing else ever would.

Leaking there is terminal: the limiter sits outside the circuit's state check, so every call dropped while the circuit is open permanently shrinks the effective limit, until all calls are rejected with ErrConcurrencyLimitReached and none can reach the circuit to ever close it again.

Also documents the obligation on the ObserverFactory contract, since it applies to any middleware claiming resources before delegating.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a permit leak in the ConcurrencyLimiter middleware when the wrapped ObserverFactory rejects a call (returns an error). Without releasing the already-acquired permit on that error path, the limiter can permanently drain its capacity—especially when placed outside a circuit breaker’s state check—eventually preventing any calls from reaching the circuit so it can recover.

Changes:

• Release the semaphore permit when the inner ObserverFactory returns an error after the limiter has acquired a permit.
• Add a regression test covering both blocking and non-blocking limiter variants for the inner-error path.
• Document the ObserverFactory contract obligation to clean up any claimed resources before returning an error.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
limiter.go	Releases acquired permits when the inner factory errors, preventing terminal capacity leaks.
limiter_test.go	Adds a regression test ensuring permits are released when the inner factory rejects calls.
hoglet.go	Documents the ObserverFactory error-path responsibility to release claimed resources.

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