Fix falsy-zero requestId bugs in debounce guard and cancellation handling

[dumbCodesOnly]

Summary

Two separate call sites in packages/core-internal/src/shared/protocol.ts treat a valid requestId/relatedRequestId of 0 as absent, because JS treats 0 as falsy. Request IDs start at 0 (_requestMessageId = 0), so this is a reachable bug in normal operation, not a theoretical edge case.

Site 1 — #2117: debounce guard wrongly skips zero-id

In _notificationViaCodec:

const canDebounce = debouncedMethods.includes(notification.method)
    && !notification.params
    && !options?.relatedRequestId;

!options?.relatedRequestId is true both when relatedRequestId is undefined and when it's 0. A notification tied to request 0 could be incorrectly treated as debounce-eligible and silently coalesced with unrelated notifications of the same method — when it should never be debounced.

Fixed to options?.relatedRequestId === undefined.

Site 2 — #2283: cancellation notification dropped for request id 0

In _oncancel:

private async _oncancel(notification: CancelledNotification): Promise<void> {
    if (!notification.params.requestId) {
        return;   // drops the notification when requestId is 0
    }
    ...
}

A notifications/cancelled message for request id 0 was silently ignored, so the abort controller for that request never fired.

Fixed to explicitly check for undefined/null.

Testing

Added regression tests in packages/core-internal/test/shared/protocol.test.ts:

• relatedRequestId: 0 must never be debounce-coalesced
• notifications/cancelled with requestId: 0 must abort the correct request's controller

Both new tests fail against the pre-fix code and pass with the fix applied.

Fixes #2117
Fixes #2283

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