Fix client read loop spinning and stream message reading

[GerardGao]

This PR fixes two issues in the client read loop.

1. Exit read loop on permanent connection error

Client.readUntilClosed ignores read errors, so once the peer closes the connection the loop spins at ~200M Read calls/sec (reproduced: 25M calls in 100ms), pinning a CPU until the client is closed.

The connection is unusable after a read error anyway: the peer may have closed it, or a stream transport may be out of sync. Exiting the loop lets the existing timeout collector terminate in-flight transactions, so the client stays consistent with RFC 5389 RTO semantics.

2. Read large and fragmented messages on streams

The client read a single 1024-byte buffer per message, which:

• truncated messages larger than the buffer, and
• desynchronized stream connections (TCP/TLS) whenever the server fragmented or coalesced messages.

Datagram connections now use a buffer sized to the largest possible STUN message (20-byte header + 65535-byte body). Stream connections are detected via LocalAddr and messages are reassembled from the 16-bit length field before decoding, handling fragmentation, coalescing and a final message delivered together with EOF.

Tests

Added regression tests covering both fixes:

• read loop exits on EOF (with and without a logger), transactions still time out afterward
• large messages on TCP and datagram connections
• fragmented messages on TCP
• coalesced messages on TCP
• final message delivered together with EOF
• malformed stream data stops the loop
• strict mode rejects reserved message type 0

All modified lines are covered by tests; the full suite, -race, golangci-lint and the wasm build pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.61%. Comparing base (d832df6) to head (69d6ac0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #289      +/-   ##
==========================================
+ Coverage   66.43%   66.61%   +0.18%     
==========================================
  Files          27       27              
  Lines        2124     2127       +3     
==========================================
+ Hits         1411     1417       +6     
+ Misses        700      698       -2     
+ Partials       13       12       -1     

Flag	Coverage Δ
go	66.61% <100.00%> (+0.18%)	⬆️

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