fix: rebuild libmongoc with SecureTransport for MongoDB Atlas TLS

[datlechin]

Summary

• MongoDB Atlas connections via mongodb+srv:// failed with "TLS handshake failed: certificate verify failed" because libmongoc was built with statically-linked OpenSSL that had no default CA certificate bundle
• Rebuilt libmongoc with -DENABLE_SSL=DARWIN (SecureTransport) instead of OpenSSL, so TLS certificate verification uses the macOS system Keychain automatically
• Removed OpenSSL build step from build-libmongoc.sh (no longer needed — libssh2 has its own separate build script)
• Updated MongoDB plugin linker flags: replaced -lssl -lcrypto with -framework Security -framework CoreFoundation

Test plan

• MongoDB Atlas connection via mongodb+srv:// URI succeeds without providing CA cert
• MongoDB local connection without TLS still works
• SSH tunnel connections still work (uses separate OpenSSL from libssh2)
• Build succeeds for both Debug and Release