Fix /pairings handler rejecting ListPairings with "tlv8: EOF"

[hughobrien]

Summary

The /pairings handler decodes requests into a struct that marks Identifier (tag 1) as required. A ListPairings request contains only Method (tag 0) and State (tag 6); it has no Identifier. The tag-keyed reader returns io.EOF for the missing required field, so tlv8.UnmarshalReader fails and the handler responds HTTP 400, logging tlv8: EOF.

iOS issues ListPairings to enumerate an accessory's paired controllers. With the request always failing, the controller set never reconciles; ListPairings, RemovePairing, and pair-verify are retried indefinitely.

Change

Mark Identifier optional. 340cbe4 (#21) previously made PublicKey and Permission optional but left Identifier required, so ListPairings still failed. Fixes #21 and #44.

Captured requests

pairings() was instrumented to log ContentLength, the read body length and error, and the raw bytes immediately before tlv8.UnmarshalReader. Observed during pairing with a real controller:

method=POST CL=6  bodyLen=6  readErr=<nil> body=000105060101
tlv8: EOF
method=POST CL=81 bodyLen=81 readErr=<nil> body=000103060101012437463141...0b0101   (AddPairing, accepted)
method=POST CL=44 bodyLen=44 readErr=<nil> body=000104060101012443433838...          (RemovePairing, accepted)

The failing body is fully read (bodyLen=6, readErr=<nil>); it is not empty or truncated. Decoding 000105060101:

00 01 05   Method (tag 0), len 1 = 0x05  (ListPairings)
06 01 01   State  (tag 6), len 1 = 0x01
           no Identifier (tag 1)

Test

TestPairingsHandlerRequests replays the captured bodies against the handler.

master:

=== RUN   TestPairingsHandlerRequests
INFO pairings.go:41: tlv8: EOF
    pairings_test.go:56: ListPairings: handler returned HTTP 400, want 200
--- FAIL: TestPairingsHandlerRequests

this PR:

=== RUN   TestPairingsHandlerRequests
--- PASS: TestPairingsHandlerRequests (0.00s)

[hughobrien]

Hello, while the above is Claude I am an actual human (promise!) and have been having a lot of fun with your library in some home automation projects (A, B). I have an Apple TV and 2x Homepods so I may be encountering this more than most. Let me know if I can provide any additional context.