fix(node/child_process): ensure stdio pipes exist when spawn fails

[gopoto]

Closes #18667

When spawning a process using the Node child_process polyfill and stdio: "pipe",
the runtime would only set up stdin, stdout, and stderr after Deno.Command.spawn()
succeeded. If the underlying command failed to spawn (e.g. permission denied or not found),
these streams remained null, leading consumers like esbuild (used by Vite) to throw
Cannot read properties of null (reading 'on') when attaching handlers.

This patch:

• Pre‑allocates fallback PassThrough streams for stdin, stdout, and stderr when
  stdio is 'pipe' so that they’re always non‑null, even if spawn fails. Node guarantees
  the same for pipe streams.
• Adjusts the constructor to overwrite these stubs with real streams once the child is spawned.
• Adds a unit test to tests/unit_node/child_process_test.ts exercising a spawn failure with
  stdio: "pipe" to ensure the streams are present and handlers can be attached without throwing.

Formatting and linting pass on the touched files.

Notes

The entire test suite requires a fresh deno build to pick up the polyfill changes;
running deno test with the prebuilt binary will still show the old behaviour.

Checklist

• Added a unit test reproducing the failure.
• Ran deno fmt / deno lint on modified files.
• Referenced the issue being fixed.

Please let me know if anything needs tweaking.

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This PR was generated by an AI system in collaboration with maintainers: @dsherret