fix(mcp): keep a snapshot open ahead of later calls on the same connection (#3661)

open_binary read snapshot_path before it entered the session's lifecycle
order, so a current_document or close_binary dispatched after it could
run first: the read returned Evidence for the previous target and the
close left the snapshot target open. The session now loads the snapshot
inside the open's own turn.

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

src/application/binary/BinarySessionOpen.ts

@@ -19,6 +19,11 @@ import { err, type Result } from "../../domain/result.js";
 import type { SessionProviderRoute } from "./SessionProviderRouter.js";
 import { SessionProviderRouter } from "./SessionProviderRouter.js";
 
+/** Read a staged snapshot once the open holds its place in the session order. */
+export type AnalysisSnapshotLoader = () => Promise<
+  Result<AnalysisSnapshot, AnalysisError>
+>;
+
 export interface BinarySessionOpenOptions {
   readonly signal?: AbortSignal;
   readonly targetKind?: BinaryTarget["kind"];
@@ -26,6 +31,11 @@ export interface BinarySessionOpenOptions {
   readonly architecture?: BinaryArchitecture;
   readonly entryPoint?: string;
   readonly snapshot?: AnalysisSnapshot;
+  /**
+   * Load the snapshot inside the open's turn instead of before it, so calls
+   * dispatched after the open cannot overtake its file read.
+   */
+  readonly loadSnapshot?: AnalysisSnapshotLoader;
   readonly providerId?: AnalysisProviderSelector;
 }