fix(stream): route to respective fd

[aeppling]

Summary

• Fix fd routing in streaming path: filtered output now goes to the same fd as the source line (stderr→stderr, stdout→stdout) instead of all going to stdout
• Affects cargo build, cargo test, cargo check, cargo fmt, tsc : all commands using FilterMode::Streaming
• flush() and on_exit() also route to the correct fd via filter_fd_is_stderr tracking

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk cargo build 1>/tmp/out 2>/tmp/err → stdout=0B, stderr has filtered warnings
• Manual testing: rtk cargo check 1>/tmp/out 2>/tmp/err → stdout=0B, stderr has filtered warnings
• Manual testing: rtk cargo test 1>/tmp/out 2>/tmp/err → summary on stdout, stderr=0B (noise filtered)
• Exit code propagation: rtk cargo build; echo $? → 0

[pszymkowiak]

[w] wshm · Automated triage by AI

📊 Automated PR Analysis

🐛 Type	bug-fix
🟡 Risk	medium

Summary

Fixes file descriptor routing in the streaming filter path so that filtered output is written to the same fd as its source (stderr lines to stderr, stdout lines to stdout) instead of everything going to stdout. Also updates flush() and on_exit() to route to the correct fd using a tracking variable. The test is renamed and updated to verify correct fd routing.

Review Checklist

• Tests present
• Breaking change
• Docs updated

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Analyzed automatically by wshm · This is an automated analysis, not a human review.

[KuSh]

LGTM 🚀️