fix: harden dynamic forwarding, ssh-agent, and EC key parsing

[GT-610]

Summary

This PR contains three small hardening fixes from ServerBox's fork, kept together:

Dynamic SOCKS forwarding

• Preserve half-close semantics while streaming:
  • client EOF closes the remote sink
  • remote EOF destroys the local socket
  • EOF before streaming still closes the SOCKS connection
• Guard against concurrent dial attempts for the same SOCKS request.
• Cancel the handshake timer before dialing and clean up a tunnel if the connection was closed while the dial was in flight.
• Decode SOCKS domain names with allowMalformed: true so malformed UTF-8 cannot crash request parsing.
• Cap SOCKS handshake buffering at 32 KiB to avoid unbounded memory growth.

SSH agent

• Validate that fallback RSA signing produces an SSHRsaSignature with the requested signature type.
• Reject zero-length and oversized agent frames (256 KiB max) to avoid malformed/unbounded frames.

EC private key parsing

• Parse EC PRIVATE KEY curve OIDs when present.
• Validate embedded EC public points by deriving the expected public key from the private scalar.
• Preserve UnsupportedError for unsupported key types/curves instead of wrapping it as SSHKeyDecodeError.
• Decode OpenSSH key comments with allowMalformed: true to tolerate malformed comments.

Test plan

• dart test test/src/socket/dynamic_forward_io_test.dart
• dart test test/src/ssh_agent_test.dart
• dart test test/src/ssh_key_pair_test.dart test/src/ssh_pem_test.dart
• dart test test/src/socket/dynamic_forward_io_test.dart test/src/ssh_agent_test.dart test/src/ssh_key_pair_test.dart test/src/ssh_pem_test.dart (50 tests)
• dart analyze lib/src/dynamic_forward_io.dart lib/src/ssh_agent.dart lib/src/ssh_key_pair.dart test/src/socket/dynamic_forward_io_test.dart test/src/ssh_agent_test.dart

[codecov]

Codecov Report

❌ Patch coverage is 69.11765% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.59%. Comparing base (43a8980) to head (4e707e1).

Files with missing lines	Patch %	Lines
lib/src/ssh_key_pair.dart	56.00%	11 Missing ⚠️
lib/src/ssh_agent.dart	41.66%	7 Missing ⚠️
lib/src/dynamic_forward_io.dart	90.32%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
+ Coverage   54.52%   54.59%   +0.06%     
==========================================
  Files          66       66              
  Lines        5357     5409      +52     
==========================================
+ Hits         2921     2953      +32     
- Misses       2436     2456      +20     

Flag	Coverage Δ
unittests	54.59% <69.11%> (+0.06%)	⬆️

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Files with missing lines	Coverage Δ
lib/src/dynamic_forward_io.dart	95.40% <90.32%> (-1.31%)	⬇️
lib/src/ssh_agent.dart	85.24% <41.66%> (-3.95%)	⬇️
lib/src/ssh_key_pair.dart	77.41% <56.00%> (-1.91%)	⬇️

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