testing: Close POSIX result pipes on FIFO EOF

[eleanorjboyd]

Summary

• replace the POSIX FIFO net.Socket reader with a nonblocking reader that explicitly detects writer disconnect and emits reader completion
• preserve buffered result delivery during cancellation while cleaning up FIFO descriptors and files exactly once
• return an owned result-pipe handle so pytest and unittest adapters dispose resources after the drain timeout fallback
• add real POSIX FIFO coverage for buffered payloads, empty writers, cancellation, and adapter cleanup

Why

A net.Socket created from a nonblocking FIFO descriptor receives data but does not emit end or close when the FIFO writer exits. After the cancellation drainage change, test execution therefore waited for the five-second fallback on every affected run.

The new reader drains the descriptor directly and settles onClose after EOF, avoiding the delay without reintroducing dropped buffered results.

Fixes #26071

[Copilot]

Pull request overview

This PR fixes a POSIX FIFO EOF detection gap in the test result named-pipe reader that caused test runs to regularly wait for the 5s RESULT_PIPE_DRAIN_TIMEOUT_MS fallback after the Python subprocess exited. It replaces the FIFO net.Socket reader with a direct FD reader that can reliably settle onClose on writer disconnect, and updates adapters to explicitly dispose result-pipe resources after the drain/timeout window.

Changes:

• Replace the POSIX FIFO reader implementation to detect writer EOF and fire close reliably (FifoMessageReader).
• Change startRunResultNamedPipe() to return an owned { name, dispose } handle and have pytest/unittest execution adapters dispose it after drain/timeout.
• Update/extend unit tests to cover POSIX FIFO reader behavior and adapter disposal during cancellation.

Show a summary per file

File	Description
src/client/common/pipes/namedPipes.ts	Adds a POSIX FIFO FD-based message reader and wires it into createReaderPipe().
src/client/testing/testController/common/utils.ts	Changes startRunResultNamedPipe() to return an owned handle with a dispose() method.
src/client/testing/testController/pytest/pytestExecutionAdapter.ts	Uses resultPipe.name and disposes the result pipe after drain/timeout.
src/client/testing/testController/unittest/testExecutionAdapter.ts	Uses resultPipe.name and disposes the result pipe after drain/timeout.
src/test/common/pipes/namedPipes.unit.test.ts	Adds POSIX FIFO tests for buffered payloads, empty writer, and cancellation close.
src/test/testing/testController/pytest/pytestExecutionAdapter.unit.test.ts	Updates stubs to match the new { name, dispose } result-pipe return type.
src/test/testing/testController/unittest/testExecutionAdapter.unit.test.ts	Updates stubs to match the new { name, dispose } result-pipe return type.
src/test/testing/testController/testCancellationRunAdapters.unit.test.ts	Asserts the adapter disposes the result-pipe handle during cancellation flows.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Lite