perf: Reduce per-request allocations, eliminate regex recompilation, and modernize to async I/O

[Copilot]

Systematic performance audit and fixes across the HttpMock request pipeline, targeting allocations and CPU work on every request.

Critical — per-request regex and I/O

• Regex caching: EndpointMatchingRule.MatchPath was constructing a new Regex for every handler×request match attempt. Now cached in a static ConcurrentDictionary<string, Regex> with RegexOptions.Compiled.
• FileResponseBody: Range regex made static readonly; single Match() replaces IsMatch() + Match(). File reads now seek to offset instead of allocating the entire file into memory.
• Async I/O: IRequestProcessor.OnRequest returns Task; body reading uses ReadToEndAsync. HttpServer listener loop uses GetContextAsync(), removing the dedicated Thread.

High — hot-path allocation reduction

• HeaderMatch: TryGetValue O(1) lookup with case-insensitive fallback replaces O(H×R) .Any() scan.
• RequestMatcher: Single foreach with early exit replaces three chained .Where() + FirstOrDefault().
• EndpointMatchingRule: Short-circuits on method mismatch before parsing query string or headers.
• _handlers.Count() (LINQ, O(n)) → .Count (O(1))
• header.Key.ToLowerInvariant() → string.Equals(..., OrdinalIgnoreCase)
• new byte[0] → Array.Empty<byte>()
• Debug log calls guarded with IsEnabled(LogLevel.Debug) to avoid SanitizeForLog allocations
• Pre-sized Dictionary in HandleContext using header count

Threading

• HttpServerFactory: Double-check lock pattern — BuildServer() (which calls Start() + TCP polling) now runs outside _serverLock.
• Fire-and-forget HandleContextAsync gets .ContinueWith(OnlyOnFaulted) error handler.

Breaking change

IRequestProcessor.OnRequest signature changed from void to Task. Unit tests updated to async Task accordingly.

[hibri]

@copilot resolve the merge conflicts in this pull request