fix(nbd): downgrade broken-pipe socket errors from ERROR to WARN

[AdaAibaby]

Problem

When a sandbox VM terminates, the NBD dispatch loop has in-flight goroutines that try to write responses back to the kernel via the Unix socket. Since the socket is already closed, these writes fail with EPIPE ("broken pipe"). The first failure is sent to the fatal channel; all subsequent concurrent goroutines hit the default: branch and log at ERROR:

ERROR nbd error cmd write  {error: write unix @- write: broken pipe, nbd_op: write, ...}
ERROR nbd error cmd write  {error: write unix @- write: broken pipe, nbd_op: write, ...}
...

This generates a burst of ERROR-level log noise on every normal sandbox shutdown, making it harder to spot real failures.

Fix

Add isSocketClosed() to detect syscall.EPIPE and io.ErrClosedPipe and emit WARN instead of ERROR in the default: branches of cmdRead, cmdWrite, and cmdWriteZeroes. Unexpected non-EPIPE errors remain at ERROR.

This is the write-side counterpart of the read-side ErrObjectNotExist downgrade already in dispatch.go.

Test plan

• Deploy to staging and confirm EPIPE response errors appear as WARN in logs during sandbox shutdown
• Confirm unexpected write errors (non-EPIPE) still appear as ERROR

[gemini-code-assist]

Code Review

This pull request introduces a helper function isSocketClosed to detect when an NBD Unix socket is closed by a peer, and updates the cmdRead, cmdWrite, and cmdWriteZeroes dispatch functions to log these expected socket closures as warnings rather than errors. The review feedback suggests a refactoring to dynamically assign the logging function to a variable, which avoids duplicating the extensive list of logging fields across the if/else branches in each of these methods.

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