encoding/base32: Add custom alphabet test case

Add test case to verify custom alphabet support. The test uses a
decimal-uppercase alphabet (0-9, A-V) to test both encoding and decoding
with custom tables, including validation. This ensures the encode and
decode functions work correctly with custom encoding tables and
validation functions as documented.

core/encoding/base32/base32_test.odin

@@ -148,3 +148,78 @@ test_base32_roundtrip :: proc(t: ^testing.T) {
 		testing.expect(t, bytes.equal(decoded, transmute([]byte)input))
 	}
 }
+
+@(test)
+
+test_base32_custom_alphabet :: proc(t: ^testing.T) {
+	custom_enc_table := [32]byte{
+		'0', '1', '2', '3', '4', '5', '6', '7',
+		'8', '9', 'A', 'B', 'C', 'D', 'E', 'F',
+		'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N',
+		'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V',
+	}
+
+	custom_dec_table: [256]u8
+	for i := 0; i < len(custom_enc_table); i += 1 {
+		custom_dec_table[custom_enc_table[i]] = u8(i)
+	}
+
+	/*
+	custom_dec_table := [256]u8{
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0x00-0x0f
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0x10-0x1f
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0x20-0x2f
+		0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  0,  0,  0,  0,  0,  0, // 0x30-0x3f ('0'-'9')
+		0, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, // 0x40-0x4f ('A'-'O')
+	 25, 26, 27, 28, 29, 30, 31,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0x50-0x5f ('P'-'V')
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0x60-0x6f
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0x70-0x7f
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0x80-0x8f
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0x90-0x9f
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0xa0-0xaf
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0xb0-0xbf
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0xc0-0xcf
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0xd0-0xdf
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0xe0-0xef
+		0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, // 0xf0-0xff
+	}
+	*/
+
+	custom_validate :: proc(c: byte) -> bool {
+		return (c >= '0' && c <= '9') || (c >= 'A' && c <= 'V') || c == byte(PADDING)
+	}
+
+	cases := [?]struct {
+		input: string,
+		enc_expected: string,
+	}{
+		{"f", "CO======"},
+		{"fo", "CPNG===="},
+		{"foo", "CPNMU==="},
+	}
+
+	for c in cases {
+		// Test encoding
+		encoded := encode(transmute([]byte)c.input, custom_enc_table)
+		testing.expect(t, encoded == c.enc_expected)
+
+		// Test decoding
+		decoded, err := decode(encoded, custom_dec_table, custom_validate)
+		defer if decoded != nil {
+			delete(decoded)
+		}
+
+		testing.expect_value(t, err, Error.None)
+		testing.expect(t, bytes.equal(decoded, transmute([]byte)c.input))
+	}
+
+	// Test invalid character detection
+	{
+		input := "WXY=====" // Contains chars not in our alphabet
+		output, err := decode(input, custom_dec_table, custom_validate)
+		if output != nil {
+			delete(output)
+		}
+		testing.expect_value(t, err, Error.Invalid_Character)
+	}
+}