Replace no-op spawn handler with native child_process.spawn bridge

[chubes4]

Summary

• Replace createNoopSpawnHandler (which immediately exits with code 1) with createNativeSpawnHandler that bridges to Node.js child_process.spawn
• This restores proc_open, exec, shell_exec, popen, and system functionality in Studio's WP-CLI environment
• php-wasm's setSpawnHandler already supports receiving a child_process.spawn-compatible function directly — the infrastructure was there, Studio just opted out

Fixes #3044

Motivation

The no-op handler silently breaks any PHP code that spawns processes. This affects:

1. WP-CLI internals — wp --info can't detect MySQL/MariaDB binary, wp db export fails
2. Automattic's php-mcp-client — uses proc_open to spawn MCP servers, completely broken in Studio
3. Plugin/theme code — any WordPress code calling exec() or proc_open() silently gets nothing back

Studio is a local development tool where PHP already has host filesystem access via createNodeFsMountHandler mounts. Process spawning is consistent with that existing trust model.

Test plan

Tested against a local Studio site by building the CLI from source (npm run cli:build) and comparing patched vs unpatched behavior:

wp --info (the command from #2519):

Field	Unpatched (no-op)	Patched (native spawn)
MySQL binary	(empty — proc_open silently failed)	/opt/homebrew/bin/mariadb
MySQL version	(empty)	mariadb from 12.1.2-MariaDB, client 15.2 for osx10.20 (arm64)

shell_exec test:

# Patched:   "hello"
# Unpatched: (empty output)

proc_open test (what php-mcp-client uses):

# Patched:   "proc_open_works"
# Unpatched: (empty output)

Regression — basic WP-CLI commands still work:

• wp --version → WP-CLI 2.12.0
• wp core version → 6.9.4
• wp --info → full output with MySQL binary now detected

AI disclosure

Per WordPress AI Guidelines:

AI assistance: Yes
Tool: Claude Code (Claude Opus 4.6)
Used for: Researching the php-wasm spawn handler API, tracing the SpawnHandler type through @php-wasm/universal and @php-wasm/util to confirm child_process.spawn compatibility, writing the replacement, and running verification tests. All code reviewed and validated by the author.

[wpmobilebot]

📊 Performance Test Results

Comparing 8eba4b7 vs trunk

app-size

Metric	trunk	8eba4b7	Diff	Change
App Size (Mac)	1285.90 MB	1285.90 MB	0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	8eba4b7	Diff	Change
load	1566 ms	1646 ms	+80 ms	🔴 5.1%

site-startup

Metric	trunk	8eba4b7	Diff	Change
siteCreation	8123 ms	8119 ms	4 ms	⚪ 0.0%
siteStartup	4184 ms	4326 ms	+142 ms	🔴 3.4%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[chubes4]

We need this to hit MCP via WP-CLI.

[chubes4]

CI Note

The lint (typecheck) failure is expected — this PR depends on WordPress/wordpress-playground#3481, which adds the nativeSpawn option to RunCLIArgs. Once that playground PR is merged and a new @wp-playground/cli version is published, we'll bump the dependency here and CI will pass.

[gcsecsey]

Thank you for creating this fix @chubes4, these changes make sense to me.

I patched the changes from WordPress/wordpress-playground#3481, and tested running the wp --info command and checking the output about the installed MySQL.

trunk	this branch