fix: prevent UTF-8 corruption when stdout and stderr interleave

[me2seeks]

Issue for this PR

Closes#36655

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Bash output currently merges raw stdout and stderr bytes before UTF-8 decoding. If stdout ends a chunk in the middle of a multibyte character and stderr writes before the remaining bytes arrive, both pipes are fed through the same decoder state and the output contains replacement characters.

This changes text output consumers to decode stdout and stderr with separate streaming decoders, then merge the decoded text. Raw combined output remains available for byte-oriented callers. The same text-safe path is used by the current shell tool, direct session shell execution, and the Core Bash implementation.

Cross-stream ordering remains best-effort, as it was before this change.

How did you verify your code works?

• Added a deterministic stream test that splits one emoji across stdout chunks with a stderr chunk in between.
• Added subprocess regression coverage for AppProcess, the shell tool, and direct session shell execution.
• Ran the affected Core and OpenCode test files.
• Ran Core and OpenCode typechecks serially with Turbo concurrency limited to 1.
• Ran Prettier and Oxlint on all changed files.

Screenshots / recordings

Not applicable; this changes captured shell text without changing the UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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