Fix set_ca_cert_store() to skip system certs like set_ca_cert_path()

[solarispika]

Problem

set_ca_cert_path() and set_ca_cert_store() behave inconsistently regarding system certificate loading:

Method	Skips system certs
set_ca_cert_path()	Yes
set_ca_cert_store()	No

Both APIs conceptually do the same thing: "use these CA certs for verification." However, when using set_ca_cert_store() for certificate pinning, system certs are still loaded in load_certs(), defeating the purpose of pinning.

Root Cause

In load_certs():

if (!ca_cert_file_path_.empty()) {
  // load from file - skips else branch
} else if (!ca_cert_dir_path_.empty()) {
  // load from dir - skips else branch
} else {
  // load system certs - ALWAYS runs when using set_ca_cert_store()
}

set_ca_cert_path() sets ca_cert_file_path_, taking the first branch and skipping system cert loading.

set_ca_cert_store() sets ca_cert_store_ (since #2217 fix), but load_certs() doesn't check it, so it falls through to the else branch where system certs are added to the user's custom store.

Fix

Check ca_cert_store_ in load_certs():

    } else if (!ca_cert_store_) {  // Was: } else {
      // load system certs
    }

This makes set_ca_cert_store() behave consistently with set_ca_cert_path().

Test

Added SSLClientTest.SetCaCertStoreSkipsSystemCerts_Online to verify system certs are not loaded when custom store is set.

[yhirose]

@solarispika thanks for finding the problem!