fix: Pipe process.stdout.write through Buffer to fix node incompatibility

[lindskogen]

What does this PR do?

Happened upon a bug in bun, where it currently ignores the encoding parameter of process.stdout.write. This PR makes it so it uses Buffer.from to encode the data.

How did you verify your code works?

• I used claude code to generate a few test cases and built bun locally to verify compatibility with node.

[claude]

Claude Code Review

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[coderabbitai]

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• test/js/node/process/process-stdio.test.ts

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Walkthrough

Adds string-to-Buffer normalization for non-UTF8 encodings in file stream write paths and introduces regression tests exercising process.stdout.write behavior with various encodings ("binary"/"latin1", "hex", and default UTF-8).

Changes

Cohort / File(s)	Summary
File Stream Write Normalization src/js/internal/fs/streams.ts	Inserted conversion of string data to Buffer.from(data, encoding) when encoding is provided and is not "utf8", "utf-8", or "buffer" in three write paths (_write, underscoreWriteFast, writeFast).
Process Stdout Write Tests test/js/node/process/process-stdio.test.ts	Added spawnSync-based regression tests for process.stdout.write() covering string with "binary"/"latin1", "hex", and no-encoding (default UTF-8) cases, asserting exact emitted byte sequences and exit code 0.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: fixing process.stdout.write to pipe through Buffer for encoding support, addressing a Node.js incompatibility in Bun.
Description check	✅ Passed	The description covers both required template sections: clearly explains the bug (encoding parameter being ignored) and the fix (using Buffer.from), and documents verification via test case generation and local build testing.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@test/js/node/process/process-stdio.test.ts`:
- Around line 164-204: Each test that spawns a process (the three tests using
spawnSync with bunExe and bunEnv) lacks an assertion on the child process exit
code; update each test to capture the spawnSync result's exitCode (from the
object returned by spawnSync) and add an assertion after the existing stdout
Buffer checks to ensure exitCode is 0 (or the expected success code). Locate the
three tests named "process.stdout.write(string, '%s') writes raw bytes",
"process.stdout.write(string, 'hex') decodes hex", and
"process.stdout.write(string) defaults to UTF-8" and add the exitCode assertion
immediately after the Buffer.compare / stdout checks for each spawnSync call.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 7c75a87 and 0af9fb9.

📒 Files selected for processing (2)

• src/js/internal/fs/streams.ts
• test/js/node/process/process-stdio.test.ts