fix(limits): lower the session ceiling to 462 so the layout budget holds

An owner's tab layout holds at most MAX_TAB_REFS (512) refs, counting saved web
tabs (up to MAX_WEBVIEWS, 50 by default) as well as sessions. With a ceiling of
500, session creation could still fail with HTTP 500 on the layout limit before
the capacity message. 512 - 50 = 462; a test pins the ceiling plus MAX_WEBVIEWS
inside MAX_TAB_REFS so the two limits cannot drift apart again.

Co-Authored-By: Christian Haberl <116439789+christianhaberl@users.noreply.github.com>
Co-Authored-By: Claude <noreply@anthropic.com>

src/web/schemas.ts

@@ -1981,7 +1981,7 @@ export const CaseOrderSchema = z.object({
 /** PUT /api/session-order — global tab order (ordered sessionIds), COD-131 */
 export const SessionOrderUpdateSchema = z.object({
   // Bounded defensively: ids are uuid-ish (<=100 chars) and the client pushes only
-  // its open-tab order (max sessions is 50 by default, at most 500 via CODEMAN_MAX_SESSIONS,
+  // its open-tab order (max sessions is 50 by default, at most 462 via CODEMAN_MAX_SESSIONS,
   // see MAX_CONCURRENT_SESSIONS_CEILING) — 500 leaves enough room while
   // keeping a hostile/buggy client from persisting megabytes into state.json.
   order: z.array(z.string().max(100)).max(500),