Fix PR #10 review comments: null safety, interface visibility, cookie parsing, WebSocket headers, typos

[Copilot]

Addresses 14 Copilot review comments on PR #10 (0.0.10-alpha). Fixes span correctness bugs, API design issues, and naming inconsistencies introduced by that PR.

Bug Fixes

• TaskManager: CreateTask/CreateTask<TTask> were calling _dictionary.TryGetValue(id, ...) instead of _dictionary.TryGetValue(key, ...) after lowercasing — allowing duplicate entries with different casing. RemoveTask would throw NullReferenceException/ArgumentNullException when task or task.Id is null; added early return guard.

• RequestHeaderFields — cookie parsing: c.Split('=')[1] throws on malformed cookies and misparses values containing = (e.g. base64). Now splits on ; first, then on the first = only:

  // before
  .Select(c => { var split = c.Split('='); return new Cookie(split[0], split[1]); })
  
  // after
  .SelectMany(c => c.Split(';', ...))
  .Select(c => { var i = c.IndexOf('='); return i < 0 ? null : new Cookie(c[..i], c[(i+1)..]); })
  .Where(c => c != null)

• UnitTestRandomId.HexCharacters: Substring(4) skips one extra character past the id_ prefix (length 3), leaving one hex char unvalidated. Changed to Substring("id_".Length).

API / Design Fixes

• IUriPathSegment: internal members (Id, IsHidden setter, Uri setter) on a public interface make it unimplementable from external assemblies. Removed internal modifiers.

• ResponseHeaderFields.ToString(): Sec-WebSocket-Accept was never emitted, breaking WebSocket handshake responses. Added emission when SecWebSocketAccept is set.

• ISocketContext.MaxMessageSize: Type was ulong but docs stated null means "no limit". Changed to ulong? throughout (ISocketContext, SocketContext, SocketItem, SocketManager); null now correctly represents unlimited.

Naming / Clarity

• SegmentRegexAttribute, SegmentStringAttribute, SegmentUIntAttribute: Parameter named display was passed directly as tag to path segment constructors — misleading. Renamed to tag.

• UriQuery<TParamerer>: Typo in generic type parameter name. Renamed to TParameter.

• ISockt.cs: Renamed to ISocket.cs; updated misleading doc comment referencing non-existent cancellation token and connectMessage parameters.

Test Fix

• UnitTestStatusPageManager: Expected content length of 78 was stale after the PR's HTML rendering changes produced 72. Updated inline data.