MOD-14124 Add RM_GetContextUser api (#85)

Add RM_GetContextUser api to retrieve the RedisModuleUser set via
RM_SetContextUser, allowing modules to access the user associated with
RM_Call ACL checks.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low-risk header-only API surface change; main concern is
ABI/compatibility for modules running against older Redis versions where
`RedisModule_GetContextUser` may be unavailable.
> 
> **Overview**
> Adds a new module API entrypoint, `RedisModule_GetContextUser`, to
retrieve the `RedisModuleUser` previously set on a context via
`RedisModule_SetContextUser`.
> 
> Updates `RedisModule_Init` to resolve and expose this function via
`REDISMODULE_GET_API(GetContextUser)` so modules can read the context
user (e.g., for ACL-aware `RM_Call` flows).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3064ed1. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: TalBarYakar

redismodule.h

@@ -1379,6 +1379,7 @@ REDISMODULE_API size_t (*RedisModule_MallocSizeDict)(RedisModuleDict* dict) REDI
 REDISMODULE_API RedisModuleUser * (*RedisModule_CreateModuleUser)(const char *name) REDISMODULE_ATTR;
 REDISMODULE_API void (*RedisModule_FreeModuleUser)(RedisModuleUser *user) REDISMODULE_ATTR;
 REDISMODULE_API void (*RedisModule_SetContextUser)(RedisModuleCtx *ctx, const RedisModuleUser *user) REDISMODULE_ATTR;
+REDISMODULE_API const RedisModuleUser * (*RedisModule_GetContextUser)(RedisModuleCtx *ctx) REDISMODULE_ATTR;
 REDISMODULE_API int (*RedisModule_SetModuleUserACL)(RedisModuleUser *user, const char* acl) REDISMODULE_ATTR;
 REDISMODULE_API int (*RedisModule_SetModuleUserACLString)(RedisModuleCtx * ctx, RedisModuleUser *user, const char* acl, RedisModuleString **error) REDISMODULE_ATTR;
 REDISMODULE_API RedisModuleString * (*RedisModule_GetModuleUserACLString)(RedisModuleUser *user) REDISMODULE_ATTR;
@@ -1778,6 +1779,7 @@ static int RedisModule_Init(RedisModuleCtx *ctx, const char *name, int ver, int
     REDISMODULE_GET_API(CreateModuleUser);
     REDISMODULE_GET_API(FreeModuleUser);
     REDISMODULE_GET_API(SetContextUser);
+    REDISMODULE_GET_API(GetContextUser);
     REDISMODULE_GET_API(SetModuleUserACL);
     REDISMODULE_GET_API(SetModuleUserACLString);
     REDISMODULE_GET_API(GetModuleUserACLString);