Fix WritePropertyMultiple decoder to support multiple objects per request

[imurasawa]

Fix WritePropertyMultiple decoder to support multiple objects per request

Summary

DecodeWritePropertyMultiple in Serialize/Services.cs was only decoding a single object,
despite the BACnet specification defining WritePropertyMultiple as a
SEQUENCE OF WriteAccessSpecifications — meaning a single request can target multiple objects.

This PR fixes the decoder and updates the delegate signature in BACnetClient.cs,
following the same design as ReadPropertyMultiple (RPM), which delegates to
ASN1.decode_read_access_specification. WPM now delegates to the newly added
ASN1.decode_write_access_specification, making the two implementations fully symmetric.

---

Background

In a production environment, a commercial BMS client sent WritePropertyMultiple requests
containing multiple objects to a BACnet server built with this library. The server failed
to process them.

The issue was isolated using the bacwpm tool from bacnet-stack:

BACNET_IFACE=eth0 ./bacwpm 22 2 1017 85 15 4 50.0 2 1016 85 15 4 40.0

State	Result
Before fix	Reject: Unrecognized Service
After fix	WriteProperty Acknowledged! (SimpleAck)

---

Root Cause

The BACnet specification (ASHRAE 135) defines the WritePropertyMultiple request PDU as a
SEQUENCE OF WriteAccessSpecifications, allowing multiple objects in a single request.

The existing DecodeWritePropertyMultiple decoded only the first object and ignored
any subsequent ones, causing the request to be rejected.

---

Changes

Design

Mirrors the RPM design: just as DecodeReadPropertyMultiple delegates to
ASN1.decode_read_access_specification, DecodeWritePropertyMultiple now delegates to
the newly added ASN1.decode_write_access_specification.
This follows the design philosophy of the original bacnet-stack.

Files changed (4 files)

1. Base/BacnetWriteAccessSpecification.cs (new file)

A new struct symmetric to BacnetReadAccessSpecification:

public struct BacnetWriteAccessSpecification
{
    public BacnetObjectId objectIdentifier;
    public ICollection<BacnetPropertyValue> propertyValues;
}

2. Serialize/ASN1.cs

Added decode_write_access_specification immediately after decode_read_access_specification,
using the same variable names (p_ref, propertyIdAndValues) and code style.

3. Serialize/Services.cs

Replaced the single-object decoder with a while loop, symmetric to DecodeReadPropertyMultiple:

public static int DecodeWritePropertyMultiple(BacnetAddress address, byte[] buffer, int offset, int apduLen,
    out ICollection<BacnetWriteAccessSpecification> properties)
{
    var len = 0;
    var values = new List<BacnetWriteAccessSpecification>();
    properties = null;

    while (apduLen - len > 0)
    {
        var tmp = ASN1.decode_write_access_specification(address, buffer, offset + len, apduLen - len, out var value);
        if (tmp < 0) return -1;
        len += tmp;
        values.Add(value);
    }

    properties = values;
    return len;
}

4. BACnetClient.cs

Updated the WritePropertyMultipleRequestHandler delegate signature:

// Before
public delegate void WritePropertyMultipleRequestHandler(
    BacnetClient sender, BacnetAddress adr, byte invokeId,
    BacnetObjectId objectId,
    ICollection<BacnetPropertyValue> values,
    BacnetMaxSegments maxSegments);

// After
public delegate void WritePropertyMultipleRequestHandler(
    BacnetClient sender, BacnetAddress adr, byte invokeId,
    ICollection<BacnetWriteAccessSpecification> properties,
    BacnetMaxSegments maxSegments);

---

⚠️ Breaking Changes

The signature of WritePropertyMultipleRequestHandler has changed.
Any code subscribing to OnWritePropertyMultipleRequest must be updated:

// Before
client.OnWritePropertyMultipleRequest += (sender, adr, invokeId, objectId, values, maxSegments) =>
{
    // objectId and values received separately
};

// After
client.OnWritePropertyMultipleRequest += (sender, adr, invokeId, properties, maxSegments) =>
{
    foreach (var spec in properties)
    {
        var objectId = spec.objectIdentifier;
        var values   = spec.propertyValues;
        // ...
    }
};

---

Test Environment

• Server: Raspberry Pi 4 (Linux)
• Client: Raspberry Pi 4 (separate device), bacwpm from bacnet-stack
• Verified: multiple objects in a single WPM request received and acknowledged
• Verified: concurrent operation with RPM from a commercial BMS client

---

Notes

Dependency on #157:
This fix was verified on Linux (Raspberry Pi 4). On Linux, WPM packets only reach
the server after the UDP socket fix in #157. The decoding bug itself is OS-independent
and affects Windows as well.

Note on BACnetClient.cs diff:
BACnetClient.cs was also normalized from CRLF to LF (as mentioned in #157).
To see only the functional changes, compare against
imurasawa:fix/normalize-line-endings branch.