fix(cases): show the real reason when Link Existing rejects a case

POST /api/cases/link passed a fixed 'Invalid request body' to parseBody, which replaced the
schema's own message, so a bad name and a bad folder path (a leading ~, a character a session
working directory cannot have) looked the same. Drop the override so the first validation
issue is returned.

.changeset/case-link-validation-message.md

@@ -0,0 +1,5 @@
+---
+'aicodeman': patch
+---
+
+Link Existing now says what is wrong when it rejects a case. A bad name reports "Invalid case name format" and a bad folder path reports why (not absolute, a character a session folder cannot have, ...), instead of the same "Invalid request body" for both.