Use a short OCPP UniqueId to support firmware that truncates the echo…

[loosrob]

Use a short OCPP UniqueId to support firmware that truncates the echoed id

Summary

Server-initiated OCPP 1.6 messages currently use a full 32-character GUID as the
message UniqueId:

msgOut.UniqueId = Guid.NewGuid().ToString("N");   // 32 hex chars

This PR replaces that with an 11-character id via a small helper
(NewShortUniqueId()), applied to all six existing server-initiated senders in
OCPPMiddleware.OCPP16.cs.

Motivation

Per OCPP-J, a charge point must echo the request's UniqueId back in its
CALLRESULT, and Receive16 correlates the response by looking that id up in
_requestQueue. Most chargers (including KEBA) echo the full id and work fine.

However, at least one firmware family — eNovates, seen on a Bluecorner "Curved"
charge point (firmware 70.13.0) — truncates the echoed UniqueId to 11
characters. When a 32-char GUID is sent, the truncated echo never matches the
queued id, so every server-initiated request logs:

OCPPMiddleware.Receive16 => HttpContext from caller not found / Msg: [3,"<11-char id>",{...}]

and the API caller receives {"status":"Timeout"} after 60 s even though the
charger answered correctly.

Example (sent vs. echoed, consistent across requests):

sent : df40da35a51xxxxxxxxxxxxxxxxxxxxx   (32 chars)
echo : df40da35a51                          (11 chars)

Fix

private static string NewShortUniqueId()
{
    return Guid.NewGuid().ToString("N").Substring(0, 11);
}

11 hex chars is ~44 bits of entropy — far more than enough uniqueness for these
one-at-a-time backend requests.

Compatibility

This is backward compatible. Spec-compliant chargers echo the shorter id
unchanged, so correlation continues to work exactly as before. Verified against a
live eNovates-based charge point: with the fix, server-initiated requests
(Reset, GetConfiguration, etc.) correlate correctly and the API returns the
charger's real answer instead of a false timeout.

Scope

Only OCPP.Core.Server/OCPPMiddleware.OCPP16.cs is changed: the new helper plus
six one-line replacements in the existing senders (Reset16, UnlockConnector16,
SetChargingProfile16, ClearChargingProfile16, RemoteStartTransaction16,
RemoteStopTransaction16).

Note on overlap with the GetConfiguration/ChangeConfiguration PR

If my other PR (adding GetConfiguration/ChangeConfiguration) is merged first,
this file will have moved and this PR will need a trivial rebase (the same helper
and the same one-line swaps, additionally applied to the two new senders). Happy
to rebase whenever suits — just let me know the preferred order. The two changes
are logically independent; I kept them apart so this firmware-workaround can be
accepted, gated behind a flag, or declined on its own merits.

Licensed under GPLv3, consistent with the rest of the project.

[dallmann-consulting]

Cutting a 32-character GUID down to 11 characters seems like a much higher risk of collisions. I would prefer a solution using case-sensitive characters (like Base64 does) to extend the possible range and reduce the probability of collisions. The necessary function would only require a few lines of code.
Would that be OK for you? Then I will implement that change.