[Client] Report the stdio server's exit code and stderr when it dies

[ousamabenyounes]

Fixes #332.

A stdio server dying on startup failed with "The server process closed its output; it is no longer running." - fast since #547, but without saying why. Now:

Initialization failed: The server process exited with code 7: Fatal: missing extension ext-foo

• keeps the last 8 KiB of stderr, still logged at debug level
• the exit code is reported if the process exited within 100 ms of closing its output
• a respawned process on retry starts with a clean slate

[chr-hertel]

Hi @ousamabenyounes, thanks for the patch - we missed some higher level integration tests for the client which I fixed with #540 and also catched some issues with our implementation.

Please rebase your branch on main, so your PR runs with that new testsuite - thanks already!

[chr-hertel]

Hi @ousamabenyounes, I took this over and rebased it on main - meanwhile #547 made the client fail at once when the stdio server closes its output, so the exit detection in processFiber() and the full stdout drain weren't needed anymore.

What #332 really asks for was still missing tho: the exit code and stderr in the error. So I kept your stderr tail and the reset on respawn, and attached both to the existing failure. You stay the author.

Thanks for your work on this! 🙏

[Copilot]

🟡 Changes recommended

Unbounded stderr draining can hang requests, and signal termination is incorrectly reported as exit code -1.

2 open findings

• Unbounded stderr draining can block ticks and hang requests · New
• Report signal termination instead of misleading exit code -1 · New

What changed in this PR

Improves stdio server startup diagnostics by reporting exit status and recent stderr output.

Changes:

• Buffers the latest 8 KiB of stderr.
• Reports process exit details and resets diagnostics on retries.
• Adds regression tests and updates documentation.

File	Description
src/​Client/​Transport/​StdioTransport.php	Captures stderr and determines process exit reasons.
tests/​Unit/​Client/​Transport/​StdioTransportTest.php	Tests exit reporting, buffer limits, and retries.
tests/​Integration/​IntegrationTestCase.php	Updates failure-diagnostic commentary.
CHANGELOG.md	Documents improved stdio diagnostics.

🧠 Review effort: Balanced

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[ousamabenyounes]

Hi @chr-hertel ! i was fixing it and just saw your message now so i just stopped the fix :) so happy to contribute on this project :)...