How to pass a list of locations (longitude, latitude) to a PostgreSQL/PostGIS stored function?

Hello dear PostgreSQL users

I have prepared a https://dbfiddle.uk/vOFXNgns for my question and also
list my SQL code below.

I have created a countires_boundaries table, which I intend to fill with
.poly files provided at Geofabrik:

CREATE EXTENSION IF NOT EXISTS postgis;

CREATE TABLE IF NOT EXISTS countries_boundaries (
country TEXT PRIMARY KEY CHECK (country ~ '^[a-z]{2}$'),
boundary GEOMETRY(MULTIPOLYGON, 4326) NOT NULL
);

CREATE INDEX IF NOT EXISTS countries_boundaries_index_1
ON countries_boundaries
USING GIST (boundary);

Then I am trying to add a function, which would receive a series of
locations (longitude and latitude pairs in microdegrees) and return a list
of lowercase 2-letter country codes, like "de", "pl", "lv":

CREATE OR REPLACE FUNCTION find_countries(locations BIGINT[][])
RETURNS TABLE (country TEXT) AS $$
SELECT DISTINCT enclosing_countries.country
FROM unnest(locations) AS location_array(lng, lat)
JOIN LATERAL (
SELECT country
FROM countries_boundaries
-- Convert microdegrees to degrees and check if the location lies
within the country boundary.
WHERE ST_Contains(
boundary,
ST_SetSRID(
ST_MakePoint(lng / 1000000.0, lat / 1000000.0),
4326
)
)
) AS enclosing_countries ON TRUE;
$$ LANGUAGE sql STABLE;

Unfortunately, this gives me the error:

table "location_array" has 1 columns available but 2 columns specified

I have also tried:

CREATE OR REPLACE FUNCTION find_countries(locations BIGINT[][])
RETURNS TABLE (country TEXT) AS $$
SELECT DISTINCT enclosing_countries.country
FROM unnest(locations) AS location
JOIN LATERAL (
SELECT country
FROM countries_boundaries
-- Convert microdegrees to degrees and check if the location lies
within the country boundary.
WHERE ST_Contains(
boundary,
ST_SetSRID(
ST_MakePoint(location[1] / 1000000.0, location[2] /
1000000.0),
4326
)
)
) AS enclosing_countries ON TRUE;
$$ LANGUAGE sql STABLE;

But that gives me the error:

cannot subscript type bigint because it does not support subscripting

I had even more attempts at fixing my issue, but have not succeeded yet

In the long run I am trying to call the function from an ASP.Net Core 8 app
as:

public async Task<ISet<string>> FindCountries(IEnumerable<(long lng,
long lat)> locations)
{
HashSet<string> countries = [];

await retryPolicy.ExecuteAsync(async () =>
{
await using NpgsqlConnection connection = new(connectionString);
await connection.OpenAsync();
using NpgsqlCommand command = new("SELECT country FROM
find_countries(@locations)", connection);

// convert locations into the expected format (array of BIGINT
pairs)
List<(long lng, long lat)> locationList = [.. locations];
long[][] locationArray = [.. locationList.Select(loc => new
long[] { loc.lng, loc.lat })];
command.Parameters.AddWithValue("locations", locationArray);

await using NpgsqlDataReader reader = await
command.ExecuteReaderAsync();
while (await reader.ReadAsync())
{
string countryCode = reader.GetString(0);
if (!string.IsNullOrWhiteSpace(countryCode))
{
countries.Add(countryCode);
}
}
});

return countries;
}

Best regards
Alex