event_id [...] is not related to thread main when sending a read receipt for the "main" thread

[clokep]

From @gileluard:

I'm still on read receipts for threads and I receive the following error when sending a RR on the main thread

error=event_id $... is not related to thread main

I think we might need some special logic for the main thread in get_thread_id

[gileluard]

Just retested it and on main thread we receive an error:
[POST] _matrix/client/r0/rooms/!ekRLBNvlGiHnceBzQq:threads-dev.lab.element.dev/receipt/m.read/%24j-4JjonTway1BTy2G_rCOSSC2uxrp_8Bxi3Qyum9Q-I
Parameters

{
    "thread_id": "main"
}

-> Failure

Error Domain=org.matrix.sdk Code=6 "event_id $j-4JjonTway1BTy2G_rCOSSC2uxrp_8Bxi3Qyum9Q-I is not related to thread main" UserInfo={errcode=M_INVALID_PARAM, httpResponse=<NSHTTPURLResponse: 0x6000007c2600> { URL: https://threads-dev.lab.element.dev/_matrix/client/r0/rooms/!ekRLBNvlGiHnceBzQq:threads-dev.lab.element.dev/receipt/m.read/%24j-4JjonTway1BTy2G_rCOSSC2uxrp_8Bxi3Qyum9Q-I } { Status Code: 400, Headers {
    "Access-Control-Allow-Origin" =     (
        "*"
    );
    "Cache-Control" =     (
        "no-cache, no-store, must-revalidate"
    );
    "Content-Type" =     (
        "application/json"
    );
    Date =     (
        "Tue, 11 Oct 2022 13:00:44 GMT"
    );
    Server =     (
        "Synapse/1.68.0rc1"
    );
    "Transfer-Encoding" =     (
        Identity
    );
    "access-control-allow-headers" =     (
        "X-Requested-With, Content-Type, Authorization, Date"
    );
    "access-control-allow-methods" =     (
        "GET, HEAD, POST, PUT, DELETE, OPTIONS"
    );
    "permissions-policy" =     (
        "interest-cohort=()"
    );
} }, error=event_id $j-4JjonTway1BTy2G_rCOSSC2uxrp_8Bxi3Qyum9Q-I is not related to thread main, NSLocalizedDescription=event_id $j-4JjonTway1BTy2G_rCOSSC2uxrp_8Bxi3Qyum9Q-I is not related to thread main}

Note that the request is successful if the read receipt is sent on a specific thread using the same method.