[Repo Assist] perf: eliminate unnecessary List(T) allocations in DisplayText and GetSessionListInternal

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Summary

Two small hot-path allocations eliminated: SessionInfo.DisplayText and GetSessionListInternal.

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1. SessionInfo.DisplayText — avoid List(string) per UI render

Problem: DisplayText builds a 1–3 element list then calls string.Join, allocating both a List(string) and its backing string[] on every call. This property is called every UI refresh for every visible session.

Fix: Replace with a tuple switch over the two optional segments (mid and end); only the final interpolated string is allocated, same as before.

// Before
var parts = new List(string) { prefix };
if (!string.IsNullOrEmpty(Channel)) parts.Add(Channel);
if (!string.IsNullOrEmpty(CurrentActivity)) parts.Add(CurrentActivity);
else if (...) parts.Add(Status);
return string.Join(" · ", parts);

// After
string? mid = !string.IsNullOrEmpty(Channel) ? Channel : null;
string? end = ...;
return (mid, end) switch
{
    (null, null) => prefix,
    (not null, null) => $"{prefix} · {mid}",
    (null, not null) => $"{prefix} · {end}",
    _ => $"{prefix} · {mid} · {end}",
};

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2. GetSessionListInternal — avoid double-copy on session-list snapshot

Problem: Every session-list update called new List(SessionInfo)(_sessions.Values) (copy 1) then .ToArray() (copy 2), paying two heap allocations per snapshot.

Fix: Allocate one SessionInfo[count] array, CopyTo the dictionary values directly into it, sort in-place with Array.Sort and a static lambda.

// Before: 2 allocations
var list = new List(SessionInfo)(_sessions.Values);
list.Sort(...);
return list.ToArray();

// After: 1 allocation
var result = new SessionInfo[_sessions.Count];
_sessions.Values.CopyTo(result, 0);
Array.Sort(result, static (a, b) => ...);
return result;

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Trade-offs

• No behaviour change. All existing SessionInfoTests and GetSessionList_SortsMainSessionFirst tests pass unchanged.
• static lambda in Array.Sort avoids the delegate allocation each time (compiler caches it).

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Test Status

• ✅ Build succeeded (0 errors, 0 warnings)
• ✅ Shared tests: 525 passed, 20 skipped, 0 failed
• ✅ Tray tests: 99 passed, 0 failed

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[shanselman]

Thanks for the cleanup here — the idea makes sense, but we’re going to pass on this one for now. The current code is working today, and this is mostly a micro-optimization rather than a bug fix. We’d rather avoid extra churn in the session display/list snapshot path unless we’re addressing a larger underlying issue at the same time.