[http-client-csharp] Add Tier 1 in-memory response cache to playground-server

[Copilot]

The playground's C# /generate endpoint spawns a fresh dotnet subprocess on every request, with no caching anywhere in the pipeline. Identical replays (undo/redo, share-link reloads, demo specs hit by many users) pay the full cost. This implements Item 3, Tier 1 of the perf plan: a container-local in-memory response cache.

Changes

• playground-server/GenerationCache.cs (new) — IGenerationCache + MemoryGenerationCache backed by IMemoryCache. ComputeKey(generatorName, codeModel, configuration, generatorVersion) returns a SHA-256 hex over a length-prefixed concatenation (avoids cross-boundary collisions like "Foo"+"Bar" vs "FooBar"+""). Entry Size = body byte length; backing cache SizeLimit = 256 MB; 1-hour sliding expiration.
• playground-server/Program.cs — registers AddMemoryCache + IGenerationCache singleton. Reads the generator DLL's FileVersionInfo.FileVersion once at startup and folds it into the key, so a deploy of a new binary implicitly invalidates every entry. /generate short-circuits on hit (returns cached bytes, skips the subprocess) and serializes-once-then-caches on miss. Responses carry X-Cache: HIT|MISS.
• playground-server.Tests/ (new NUnit project, 15 tests) — key determinism & format; sensitivity to each of the four components (including version → invalidation); length-prefix unambiguity; null-argument guards; get/miss/set/overwrite round-trip; SizeLimit compaction; sliding-expiration eviction.

Cache flow

var cacheKey = MemoryGenerationCache.ComputeKey(generatorName, body.CodeModel!, body.Configuration!, generatorVersion);
if (cache.TryGet(cacheKey, out var cached) && cached is not null)
{
    request.HttpContext.Response.Headers["X-Cache"] = "HIT";
    return Results.Bytes(cached.Body, cached.ContentType); // skips dotnet subprocess
}
request.HttpContext.Response.Headers["X-Cache"] = "MISS";
// ...run generator, then:
var responseBytes = JsonSerializer.SerializeToUtf8Bytes(new GenerateResponse(files), GenerateJsonContext.Default.GenerateResponse);
cache.Set(cacheKey, new CachedGenerationResponse(responseBytes, "application/json"));
return Results.Bytes(responseBytes, "application/json");

Out of scope

Tier 2 file-based cache and Items 1/2/4/5 of the issue. Generator internals are untouched; production tsp codegen is unaffected.

[jorgerangel-msft]

@copilot remove any comments pertaining to the issue. In general, we should only add comments when the code itself is not self explanatory.

[Copilot]

Removed the Item 3 / Tier 1 / perf-plan references and trimmed other inline comments where the code was self-explanatory in 48ddd6c.