[josecelano]

Summary

• Add Registar::deregister(&ServiceBinding) -> Option<RegisteredService<M>>, an additive async method that removes one registration and returns it, or returns None when the binding is not registered.
• A stopped service can then register the same final binding again. Without it, a stop-then-start on a fixed port fails with RegistrationError::DuplicateBinding.
• The registry is keyed by binding only, so the method removes whichever registration holds that binding. The doc comment tells callers to pass only a binding they registered themselves. A receipt-based API that removes only its own entry was considered and left out as too large for now; it can be built on this method later.
• Bump torrust-server-lib from 0.3.0 to 0.4.0 for the additive public API, as the previous additive API did (0.2.0 to 0.3.0).

Validation

• cargo test: 10 passed, including 4 new tests (remove and return, register again after removal, None for an unknown binding, other registrations kept). The last was mutation-proven: clearing the whole map made it fail.
• cargo fmt --check, cargo clippy --all-targets -- -D warnings, linter all
• cargo publish --dry-run

Related to torrust/torrust-tracker#2450.

[Copilot]

🟢 Approval recommended

The mutex-protected removal matches its documented contract, focused tests cover the intended behavior, and no blocking issues were identified.

No open issues 🎉

What changed in this PR

Adds Registar::deregister so stopped services can release their registrations and later register the same binding again.

Changes:

• Adds an async removal method that returns the removed service or None, with documented caller ownership expectations.
• Adds four tests covering removal, re-registration, unknown bindings, and preservation of other registrations.
• Bumps the crate version from 0.3.0 to 0.4.0.

File	Description
src/​registar.rs	Adds deregistration, API documentation, and regression tests.
Cargo.toml	Updates the crate version to 0.4.0.
Cargo.lock	Synchronizes the crate version with the manifest.

_{Review effort: Balanced}