feat(allowedpaths): guarantee Close on context expiry to prevent fd leaks

[thieman]

Summary

• Adds `WithContextClose(ctx, f)` in `allowedpaths/context_file.go` — wraps an `io.ReadWriteCloser` so `Close()` is called automatically when the context expires, preventing fd leaks if a caller omits an explicit close
• Wires it into both `OpenFile` hook sites in `interp/runner_exec.go` (child builtin path and main builtin path)
• The underlying file is closed at most once regardless of whether the context fires or the caller calls `Close()` explicitly — two `sync.Once` guards prevent double-close
• Existing `defer f.Close()` calls in builtin implementations are untouched; this is purely a safety backstop

Read-unblock behaviour by file type

Linux unit tests (`context_file_linux_test.go`) verify two things per file type: (1) that `Close()` is guaranteed to be called on context cancellation, and (2) whether a goroutine blocked in `Read()` is unblocked when the context is cancelled.

Go registers FIFOs, sockets, and character devices with its epoll poller. When `Close()` is called, the runtime calls `evict()` which wakes any goroutine blocked on that fd. Regular files are not registered with epoll but never block, so they are safe regardless.

Non-blocking file types — Read returns immediately

File type	CI result	Read outcome
Regular file	✅ PASS	Returns data then EOF immediately
`/dev/null`	✅ PASS	Returns `(0, EOF)` immediately
`/dev/zero`	✅ PASS	Returns zeros immediately
`/dev/urandom`	✅ PASS	Returns random bytes immediately
Symbolic link → regular file	✅ PASS	Identical to regular file
Directory	✅ PASS	Returns `EISDIR` immediately
Block device (`/dev/sda` etc.)	⏭️ SKIP	No accessible block device in GH Actions; expected to return block data immediately

Blocking file types — Read blocks; context cancel must unblock it

File type	CI result	Observed error after cancel
Named pipe / FIFO	✅ PASS	`"file already closed"` — epoll `evict()` woke the goroutine
Unix socket (`net.Pipe()`)	✅ PASS	Unblocked cleanly
Filesystem Unix socket (`/tmp/*.sock`)	✅ PASS	Unblocked cleanly
`/dev/random`	✅ PASS	Didn't block (Linux ≥ 5.6 behaves like `/dev/urandom`)
`/dev/tty`	⏭️ SKIP	No controlling terminal in GH Actions; expected to unblock via epoll evict

Test plan

• `TestWithContextClose_ExplicitClose` — explicit close works normally
• `TestWithContextClose_ExplicitCloseIdempotent` — multiple explicit closes are safe
• `TestWithContextClose_ContextCancellation` — file is closed when context is cancelled
• `TestWithContextClose_ContextCancelledBeforeOpen` — file is closed when context is already done at wrap time
• `TestWithContextClose_ContextCancelThenExplicitClose_ClosedOnce` — only one close after both paths fire
• `TestWithContextClose_ErrorPropagated` — error from explicit Close is returned to caller
• `TestWithContextClose_GoroutineExitsOnExplicitClose` — no goroutine leak when context is never cancelled
• Linux file-type matrix tests — all ran or skipped with justification
• Full `./interp/... ./tests/...` suite passes on Linux, macOS, Windows

🤖 Generated with Claude Code