Fix namespace reconnect state, Engine.IO transport validation, and handshake metadata

[itsfuad]

This PR fixes four confirmed protocol and metadata bugs across the Socket.IO and
Engine.IO layers:

• removes stale namespace state after a namespace disconnect so reconnecting to
  the same namespace works on the same Engine.IO session
• rejects transport=websocket requests for polling sessions unless the request
  is a real WebSocket upgrade
• applies editResponseHeaders to CORS preflight responses as documented
• derives secure and xdomain from the actual request URL and Origin header
  instead of hardcoded values

How This Solves The Issues

Namespace reconnect

Client stored namespace sockets by namespace name but removed them by socket
id. This PR makes removal use socket.nsp.name, so 41/custom, actually clears
the namespace entry and a later 40/custom, can reconnect cleanly.

Transport mismatch validation

Engine.IO now distinguishes a valid polling-to-websocket upgrade request from an
ordinary HTTP request that merely carries transport=websocket. Non-upgrade
mismatches are rejected with the existing TRANSPORT_MISMATCH error instead of
hanging on the polling transport.

Preflight response headers

editResponseHeaders now runs before the early OPTIONS return path, so custom
response headers are applied consistently to preflight and non-preflight
requests.

Handshake metadata

Handshake metadata is now derived from the request URL and Origin header:

• secure is true for https: requests
• xdomain is only true when Origin differs from the request origin

Tests

Added regression coverage for:

• namespace reconnect after 41/custom,
• polling session transport mismatch with transport=websocket
• editResponseHeaders on OPTIONS preflight requests
• handshake secure and xdomain metadata derivation

Full test suite result on branch fix/repro-report-and-protocol-bugs:

ok | 25 passed (176 steps) | 0 failed | 0 ignored (1 step)

Command used:

/home/fuad/.deno/bin/deno test -A

[itsfuad]

@darrachequesne Could you kindly review please?

[itsfuad]

@darrachequesne Just a friendly ping. Didn't hear any feedback from you.