refactor: handle transient connections at type level

[sanity]

Background

PR #2105 introduced transient connection tracking for gateways using runtime validation. This works but could be made safer and less error-prone using Rust's type system.

Proposal

As suggested by @iduartgomez in #2105 (comment):

I think we can handle transient connections at the type level and that would make all this stuff safer and less error prone, but that will require some further changes/refactoring so will do that in a separate PR in the future

Current Implementation

Transient connections are tracked via:

• Runtime checks in ConnectionManager::register_transient()
• Manual budget tracking with RwLock<DashMap<PeerId, TransientEntry>>
• TTL expiration checked at registration time

Type-Level Approach

Use Rust's type system to enforce transient connection constraints at compile time:

• Separate connection types (e.g., Connection<Permanent> vs Connection<Transient>)
• Type-safe budget management
• Compile-time guarantees about connection lifecycle

References

• PR fix: propagate connection errors for blocked peer fallback (#2021) #2105: Introduced transient connection tracking
• Related: crates/core/src/ring/connection_manager.rs

[AI-assisted - Claude]

[github-actions]

🤖 Auto-labeled

Applied labels:

• T-enhancement (95% confidence)
• E-hard (85% confidence)
• A-networking (95% confidence)
• S-needs-design (90% confidence)

Reasoning: This is a refactor proposal to move transient connection tracking from runtime checks to type-level enforcement in Rust — an improvement to existing functionality rather than a bug or new feature, so T-enhancement fits. The change requires non-trivial, architecture-level work across connection manager and types (generics, budget tracking, lifecycle guarantees), so it's estimated E-hard. The scope is clearly in networking/connection management, so A-networking applies. The proposal impacts design decisions and calls for a separate PR and refactor, so S-needs-design is appropriate.

Previous labels: none

If these labels are incorrect, please update them. This helps improve the auto-labeling system.

[iduartgomez]

Or we can enforce at runtime but using a pseudo-smart-pointer instead of what we currently do if compile time checks are unfeasible. (I guess I meant something like that, rather than type level)