Fix ephemeral port leak: pass context to ClientSession.DialStream

[0pcom]

Summary

• ClientSession.DialStream now accepts context.Context
• When context is cancelled, the stream is closed immediately, freeing the reserved ephemeral port
• Client.DialStream passes its context to all ClientSession.DialStream calls

Root Cause

ClientSession.DialStream didn't accept a context. When the caller's deadline expired (e.g., 30s dial timeout in the setup-node), readResponse kept blocking on the yamux stream read until HandshakeTimeout fired. The ephemeral port reserved by writeRequest stayed allocated the entire time.

With 6 DMSG servers tried sequentially per dial attempt, and many concurrent route setup requests, the ~16K ephemeral port space exhausted within minutes.

Fix

A goroutine watches the context and calls dStr.Close() when cancelled. This interrupts the blocked readResponse, triggers the defer that frees the port, and returns immediately. The port is freed in milliseconds instead of waiting up to HandshakeTimeout.

Test plan

• Verify setup-node no longer gets "ephemeral port space exhausted" under load
• Verify normal DMSG stream dials still work
• CI tests pass