HtmlReporter state is not scoped per session when sessions overlap #7298

Problem

HtmlReporter keeps all its state in fields shared by every session in the process, with no session key:

• _updates (src/TUnit.Engine/Reporters/Html/HtmlReporter.cs:40) is one ConcurrentDictionary<string, TestNodeUpdateMessage>.
• OnTestSessionStartingAsync clears it (HtmlReporter.cs:124), and BuildReportData renders all of it (HtmlReporter.cs:613-614).

When MTP server mode runs two sessions in one process at the same time, the second session's start wipes the first session's results. Each report can also contain the other session's tests. The unobserved task exception queue added in #7296 follows the same scoping.

Raised during review of #7296 (#7296 (comment)).

Expected

Either key the reporter state (updates, unobserved exceptions, activity collection) by SessionUid and build each report from its own session's entries, or document that the HTML report assumes sessions do not overlap.