Move socket connection interfaces to NATS.Client.Abstractions

[mtmk]

Move the socket transport contracts INatsSocketConnection and INatsTlsUpgradeableSocketConnection to NATS.Client.Abstractions so custom transport implementers can depend on the contract without referencing Core, matching the serialization interfaces already there. The types keep the NATS.Client.Core namespace and are forwarded via TypeForwardedTo to preserve source and binary compatibility; the obsolete ISocketConnection and INatsSocketConnectionFactory stay in Core. The ABI compatibility harness is extended to cover both moved interfaces through a transient dependency compiled against an older release.

Fixes #1185

[synadia-claude-reviewer]

Claude finished @mtmk's task in 2m 22s —— View job

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Overall: The change is correct and well-structured. Follows the established pattern used for serialization interfaces. No bugs or security issues.

What looks good

• Namespace preservation (namespace NATS.Client.Core; in the Abstractions project file) maintains source compatibility so existing consumers don't need new using directives.
• TypeForwardedTo usage is correct — both INatsSocketConnection and INatsTlsUpgradeableSocketConnection are forwarded in TypeForwarders.cs, which handles binary compatibility for code compiled against older assemblies.
• Microsoft.Bcl.AsyncInterfaces version (8.0.0) is consistent with what NATS.Client.Core already specifies for netstandard2.0. The dependency is necessary since IAsyncDisposable isn't in netstandard2.0 natively.
• ABI harness is extended correctly: both interfaces are checked for assembly residency, and the transient-lib scenario (simulate old binary loading against new runtime) is exercised for INatsSocketConnection usage.

Notes / things to be aware of

System.Net.Sockets.Socket in Abstractions: INatsTlsUpgradeableSocketConnection.Socket introduces a concrete BCL networking type into what is otherwise a serialization/contract package. This is available on netstandard2.0 so it won't break builds, and preserving the interface shape is required for compat — just noting that Abstractions now has a platform-networking surface.

Factory still requires Core: The goal stated in the PR description is "custom transport implementers can depend on the contract without referencing Core." That's true for the connection side — an implementation library only needs Abstractions. However, INatsSocketConnectionFactory (the entry point for registering the custom transport via NatsOpts.SocketConnectionFactory) stays in Core because it takes NatsOpts. So the application wiring still needs Core, but this is expected and the PR is a meaningful step toward decoupling.

ABI test sends 7 bytes: MySocketConnection.SendAsync returns buffer.Length and the test asserts the return equals 7 — this is intentional and correct for a stub implementation.

No blocking issues. LGTM.

[github-actions]

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[scottf]

LGTM