feat(tcp): add RemoteListen for reverse port forwarding

[rhuss]

Summary

• Add RemoteListen method to TCPInterface for reverse port forwarding (ssh -R equivalent)
• Includes functional options (WithRemoteBindAddress, WithRemoteListenServiceID), input validation, fake client support, and comprehensive tests
• Real gRPC implementation returns Unimplemented pending upstream proto extension

Blocked on

Upstream support for reverse port forwarding / host-local endpoint proxying:

• NVIDIA/OpenShell#1633 - supervisor-proxied host-local endpoints
• NVIDIA/OpenShell#994 - sandbox ingress

This PR parks the SDK-side API surface until the backend proto RPC is available.

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[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.39%. Comparing base (e44c700) to head (c25ed2b).

Files with missing lines	Patch %	Lines
openshell/v1/tcp.go	33.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
- Coverage   89.57%   89.39%   -0.18%     
==========================================
  Files          78       78              
  Lines        4901     4932      +31     
==========================================
+ Hits         4390     4409      +19     
- Misses        345      354       +9     
- Partials      166      169       +3     

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