typescript: add managed subscription lifetimes

[Josh (joshmouch)]

Summary

Add a framework-neutral ManagedSubscriptionManager to the official TypeScript client entry point. It provides reusable ownership and lifecycle semantics above the existing subscribe and unsubscribe wire methods without adding protocol vocabulary.

The manager owns:

• one wire subscription per resource URI
• synchronous named leases for independent holders
• one event iterator per holder, attached before the subscribe request
• retention of actions that arrive during the initial subscribe round trip
• deterministic holder and reference-count inspection
• last-holder unsubscribe and manager-wide shutdown
• failed-subscribe cleanup followed by deterministic reacquisition
• entry-identity fencing so a late result cannot overwrite a replacement subscription
• rejection of incompatible delivery or view options for an already shared resource

It deliberately does not import VS Code Event, observable, URI, Disposable, DI, or workbench abstractions. Reducer-backed state, optimistic action reconciliation, and multi-host reconnect restoration remain separate SDK decisions.

Why this belongs in the SDK

Subscriptions are already an AHP protocol concept, and the official client already owns the wire subscription plus state-mirror primitives. Reference ownership, initial-roundtrip delivery, deterministic reacquisition, and final unsubscribe are generic client lifecycle rules; leaving them to each product produces multiple subscription managers above the same SDK.

Validation

• TypeScript client typecheck
• TypeScript client build
• complete TypeScript client suite: 72 passed
• focused managed-subscription lifecycle and race suite: 6 passed
• change-fragment verification: 21 fragments
• npm pack dry run includes the source, compiled JavaScript, declarations, and source maps
• git diff --check

The implementation is stateless when a subscription returns no snapshot, so it also supports notification-only resources.

[Josh (joshmouch)]

Follow-up integration is ready on the fork branch pr/typescript-managed-host-subscriptions at 7ac6302. It preserves one managed lease across real MultiHostClient transport generations, routes replay/snapshot/missing restoration into that lease, and fences stale generation events and results. The full TypeScript suite is 78/78; focused host and reconnect suites are green. I kept it out of this PR so the shared subscription lifetime can be reviewed independently. The coordinated VS Code consumer and deletion proposal is microsoft/vscode#332854.

[Connor Peet (connor4312)]

Prefer Symbol.dispose for fully RAII-style usage

[Connor Peet (connor4312)]

I suggest allowing the consumer to spec the type of state and events they're getting here.

We actually have some prior art for this in vscode sources, see these methods and IAgentSubscription https://github.com/microsoft/vscode/blob/c19945aab6c07dac9e1afd386704af16a4d212c7/src/vs/platform/agentHost/common/agentService.ts#L1046-L1054