Stop remapping RedisModule_OnLoad; add test module that exports it via version script

[roshkhatri]

Motivation

• Avoid renaming the RedisModule_OnLoad entry-point via macro substitution because it can break modules that use a linker version script to export RedisModule_OnLoad explicitly.
• Ensure valkey-server continues to load modules that export the original RedisModule_OnLoad symbol.

Description

• Stop mapping RedisModule_OnLoad to ValkeyModule_OnLoad in src/redismodule.h and add a comment explaining why the symbol must be left unmapped.
• Add a minimal test module tests/modules/hellovalkey.c that defines RedisModule_OnLoad and a version script hellovalkey.version that exports RedisModule_OnLoad.
• Update tests/modules/Makefile to build hellovalkey.so using the version script on Linux and include it in the test modules list.
• Add a unit test tests/unit/moduleapi/redismodule_onload.tcl that verifies the RedisModule_OnLoad symbol is exported (nm -D on Linux) and that the module can be loaded and unloaded by the server.

Testing

• Built the test modules including hellovalkey.so (Linux path using the version script) and the build succeeded.
• Ran the new unit test tests/unit/moduleapi/redismodule_onload.tcl which checks the exported symbol and performs MODULE LOAD / MODULE UNLOAD, and the test passed.

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Codex Task