Sandboxed PHP subprocesses may truncate stdout and stderr

Problem

The sandboxed PHP spawn handler starts piping child stdout and stderr, then calls processApi.exit() after result.exitCode without awaiting either pipeTo() promise. A sufficiently large child response can therefore close the parent-facing streams before their tails are delivered.

This was reproduced with a PHPUnit subprocess that completed 693/695 tests but lost its final test and summary output. A focused 1 MiB subprocess probe verifies the generic contract.

Root cause

In packages/php-wasm/universal/src/lib/sandboxed-spawn-handler-factory.ts, the PHP branch creates stdout/stderr pipeTo() promises but does not await them before processApi.exit().

Acceptance criteria

• The PHP subprocess exit status is retained.
• Both stdout and stderr are fully drained before the parent-facing process exits.
• A large-output regression verifies the final tail bytes in Asyncify and JSPI.

Related, already tracked

Bare child php also needs stdin forwarded into PHP.cli(). That owning API gap is already tracked by #3519 and implemented in open PR #3523; it is intentionally outside this issue.

Start in packages/php-wasm/universal/src/lib/sandboxed-spawn-handler-factory.ts and trace the PHP branch's stdout and stderr pipeTo promises through processApi.exit(). Use the focused 1 MiB subprocess probe as the regression model; done means the exit status is retained, both streams are drained, and final tail bytes are verified in both Asyncify and JSPI.