fix(mcp): enrich bad/inaccessible spaceId 404 into an actionable error (#534)

A well-formed but non-existent/inaccessible spaceId made the space-permissions
check answer with an opaque 404 ("Space permissions not found"), which the agent
could not self-correct from. Add an enrich-on-404 wrapper at the client-method
level (Variant A — no backend change) that, ONLY on that 404, replaces the server
text with a factual message naming the bad spaceId and the spaces the token can
actually see, pointing at listSpaces.

Mechanism (context.ts):
- getAccessibleSpaceIndex(): the token's accessible spaces from the single source
  of truth (/spaces), with a per-instance short-TTL cache (MCP_SPACES_CACHE_TTL_MS,
  default 60s; 0 disables) and single-flight dedup. Only a COMPLETE (untruncated)
  result is cached and only a complete result may drive the rewrite. The in-flight
  promise is nulled on BOTH resolve and reject so a transient /spaces blip is never
  memoized. Cache invalidated on every identity change, mirroring collabTokenCache
  (login() + the 401/403 reauth interceptor).
- paginateAllWithMeta(): surfaces the `truncated` flag paginateAll swallows;
  paginateAll now delegates to it (unchanged contract, getSpaces untouched).
- withSpaceAccessDiagnostics(spaceId, mcpName, fn): abort/cap wins FIRST (by the
  toolAbortSignal flag, NOT e.name — a cap may be TimeoutError/custom); only a 404
  is enrichable; FAILS OPEN (rethrows the original server error) on every source of
  uncertainty (fetch failed / !complete / spaceId present / aborted).

Wrapped only the paths where the sole 404 cause is the space membership check:
getTree, listPages (tree + recent-with-spaceId), search (with spaceId),
checkNewComments. createPage/getPageContext are deliberately NOT wrapped.
formatSpaceNotAccessible (errors.ts) composes the message (<=10 spaces + "(+N ещё)"
tail; distinct zero-spaces variant).

Tests: 11 new unit tests (stub client) covering all 9 acceptance criteria +
message shape; full mcp unit suite green (701 tests).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/mcp/src/client/comments.ts

@@ -650,10 +650,15 @@ export function CommentsMixin<TBase extends GConstructor<DocmostClientContext>>(
     // The subtree scope (parentPageId given) already INCLUDES the root node
     // itself: /pages/tree seeds getPageAndDescendants with id = parentPageId, so
     // no separate getPageRaw fetch for the parent is needed.
-    const { pages: pagesInScope, truncated } = await this.enumerateSpacePages(
-      spaceId,
-      parentPageId,
-    );
+    // #534: the enumerateSpacePages seed (`/pages/tree`) 404s for a bad or
+    // inaccessible spaceId; wrap it so that 404 becomes an actionable "spaceId
+    // not accessible" hint instead of the opaque "Space permissions not found".
+    // Only the whole enumeration is wrapped (the only 404 source here) — see the
+    // wrap-allowlist invariant on withSpaceAccessDiagnostics.
+    const { pages: pagesInScope, truncated } =
+      await this.withSpaceAccessDiagnostics(spaceId, "checkNewComments", () =>
+        this.enumerateSpacePages(spaceId, parentPageId),
+      );
 
     // 2. Fetch comments for each page, keep ones created after since
     const results: any[] = [];