gpt: key the verdict cache on the context snippet too

The cache key is the prompt and the text part's digest only, so a message
whose text was seen before reuses that verdict even when its context
(llm_context, search_context, context_augment: sender history,
attachments) differs. Add a hash of the snippet when there is one.

--- a/src/plugins/lua/gpt.lua	2026-11-01 10:00:54.784198919 -0600
+++ b/src/plugins/lua/gpt.lua	2026-11-01 10:00:54.785151850 -0600
@@ -697,7 +697,7 @@
 -- Make cache specific to all settings to avoid conflicts
 local env_digest = nil
 
-local function redis_cache_key(sel_part)
+local function redis_cache_key(sel_part, context_snippet)
   if not sel_part then
     return nil
   end
@@ -709,8 +709,15 @@
     digest:update(settings.url)
     env_digest = digest:hex():sub(1, 4)
   end
-  return string.format('%s_%s', env_digest,
+  local key = string.format('%s_%s', env_digest,
     sel_part:get_mimepart():get_digest():sub(1, 24))
+  if context_snippet and #context_snippet > 0 then
+    -- The same text with a different context (sender history,
+    -- attachments, search results) is a different question
+    local hasher = require "rspamd_cryptobox_hash"
+    key = string.format('%s_%s', key, hasher.create(context_snippet):hex():sub(1, 8))
+  end
+  return key
 end
 
 local function process_categories(task, categories)
@@ -782,7 +789,7 @@
     task:get_mempool():set_variable('gpt_result', gpt_mp_json)
   end
 
-  local cache_key = redis_cache_key(sel_part)
+  local cache_key = task:cache_get('gpt_cache_key') or redis_cache_key(sel_part)
   if cache_context and cache_key then
     lua_cache.cache_set(task, cache_key, result, cache_context)
   end
@@ -880,7 +887,10 @@
 end
 
 local function check_llm_cached(task, content, sel_part, context_snippet)
-  local cache_key = redis_cache_key(sel_part)
+  local cache_key = redis_cache_key(sel_part, context_snippet)
+  if cache_key then
+    task:cache_set('gpt_cache_key', cache_key)
+  end
 
   if not cache_key or not cache_context then
     check_llm_uncached(task, content, sel_part, context_snippet)