child_process: add tracing channel for spawn

[marcopiraccini]

This PR adds a new child_process.spawn tracing channel wrapping the spawn call. Since spawn is synchronous, this uses start, end, and error events (rather than the async variants).

Events

• tracing:child_process.spawn:start — published before the native _handle.spawn() call, with { process, options }
• tracing:child_process.spawn:end — published on successful spawn, with { process }
• tracing:child_process.spawn:error — published on spawn failure, with { process, error }

Since exec, execFile, and fork all call spawn internally, this single tracing channel covers all child process creation paths (e.g.: https://github.com/nodejs/node/blob/main/lib/child_process.js#L174)

[ShogunPanda]

LGTM!

[mcollina]

lgtm

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.71%. Comparing base (9cc7fcc) to head (082952e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/child_process.js	75.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61836      +/-   ##
==========================================
- Coverage   89.72%   89.71%   -0.02%     
==========================================
  Files         675      675              
  Lines      204797   204821      +24     
  Branches    39344    39355      +11     
==========================================
  Hits       183752   183752              
- Misses      13324    13338      +14     
- Partials     7721     7731      +10     

Files with missing lines	Coverage Δ
lib/internal/child_process.js	94.65% <75.00%> (-0.70%)	⬇️

... and 30 files with indirect coverage changes

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