Implement node:dgram over inbound UDP listeners

[guybedford]

This makes node:dgram functional over the inbound UDP connect() handler, the same way node:net's Server is virtualized over TCP listeners. Previously the module was a silent no-op stub.

• cloudflare-internal:http gains a udpPorts table seeded from the worker's declared UDP listeners, alongside tcpPorts.
• connectHandler() in cloudflare:node routes socket.protocol === 'udp' flows to the dgram.Socket bound to the port they arrived on.
• dgram.Socket.bind() claims a declared UDP port with the same rules as net.Server.listen() (EADDRINUSE/EADDRNOTAVAIL, port 0 takes the first unclaimed declared port). Each inbound flow becomes a peer whose datagrams are emitted as 'message' events with rinfo; the flow's request lives until the flow ends or the socket closes.
• send() (all Node argument forms, plus sendto and connected mode via connect()/disconnect()/remoteAddress()) delivers one datagram per call to any peer with a live flow. A v4-mapped peer on the dual-stack listener is reported as IPv4 to a udp4 socket and matched by either form.
• The platform has no unconnected UDP egress, so a datagram to a peer without a flow fails with EHOSTUNREACH in the callback. An implied bind from send()/connect() takes an ephemeral port rather than a declared listener.
• Multicast membership throws ERR_FEATURE_UNAVAILABLE_ON_PLATFORM; TTL/broadcast/buffer-size setters validate and record.

Tests: dgram-nodejs-test is rewritten to drive fake platform flows through handleAsNodeConnection (declared-port rules, message/rinfo, every send form, mapped addresses, connected mode, implicit bind, close ending flows, option validation); a new udp-dgram-node server test runs a real Node UDP client against a node:dgram echo server in workerd across flow re-creation and concurrent peers.

Compatibility: the stub accepted any call sequence silently, so nothing built on it functioned. Validation and the already-bound/not-running/is-connected errors now match Node and only affect sequences Node itself rejects, so no compat flag is added.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0.24938% with 400 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.11%. Comparing base (1a8abde) to head (f8d6e48).

Files with missing lines	Patch %	Lines
src/workerd/api/node/tests/dgram-nodejs-test.js	0.00%	379 Missing ⚠️
src/workerd/server/tests/udp-dgram-node/index.mjs	0.00%	21 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                       @@
##           gbedford/udp-remote-address    #7506      +/-   ##
===============================================================
- Coverage                        38.15%   38.11%   -0.05%     
===============================================================
  Files                              858      859       +1     
  Lines                           262910   262561     -349     
  Branches                         24388    24287     -101     
===============================================================
- Hits                            100313   100073     -240     
+ Misses                          148928   148856      -72     
+ Partials                         13669    13632      -37     

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[ThomasRubini]

I had a quick look at the code and everything seems good, although I don't think I'm the best person to review this one