fix(terminal): lower agent process priority

[primorLee]

HUMAN:

---

AGENT:

Why

CPU-heavy agent commands currently inherit the agent-server's priority and can
starve the server. The original product report and benchmarks are documented at
OpenHands/OpenHands#16720. SDK issue #4575 contains a
minimal reproduction and testable acceptance criteria for this repository.

Summary

• Prefix macOS terminal shells with /usr/sbin/taskpolicy -c utility and Linux
  terminal shells with an absolute nice -n 10 path, covering standalone tmux,
  pooled tmux, and the PTY subprocess backend.
• Start the Windows PowerShell backend with BELOW_NORMAL_PRIORITY_CLASS and
  verify that a real child command inherits it.
• Preserve startup on Linux systems without nice, and support
  OH_TERMINAL_PROCESS_PRIORITY=none for priority-sensitive benchmarks or
  development servers.
• Add platform mapping, fallback, opt-out, and real child-process inheritance
  tests.

Issue Number

Fixes #4575

Original product issue: OpenHands/OpenHands#16720

How to Test

I completed the repository build steps on Windows (GNU Make is unavailable in
this checkout, so I ran the exact build target commands):

uv sync --dev
uv run pre-commit install

Windows end-to-end verification launches a real command through the persistent
PowerShell terminal; that child reads its own OS priority through psutil:

uv run pytest tests/tools/terminal/test_windows_terminal.py::test_terminal_process_runs_below_normal_priority -q --timeout=30
collected 1 item
tests\tools\terminal\test_windows_terminal.py . [100%]
1 passed in 1.88s

Platform policy, missing-nice, and opt-out coverage on the same checkout:

uv run pytest tests/tools/terminal/test_process_priority.py -q
tests\tools\terminal\test_process_priority.py .......ss [100%]
7 passed, 2 skipped in 0.06s

The two skips are the real Linux tmux/subprocess inheritance cases. They are
committed in this PR for the Ubuntu tools job; this machine has neither WSL nor
a Linux container runtime. The assertions use a unique output marker and compare
against the runner's actual niceness rather than assuming a zero baseline.

I also checked every changed POSIX file with Linux type stubs:

uv run pyright --pythonplatform Linux \
  openhands-tools/openhands/tools/terminal/terminal/process_priority.py \
  openhands-tools/openhands/tools/terminal/terminal/tmux_terminal.py \
  openhands-tools/openhands/tools/terminal/terminal/tmux_pane_pool.py \
  openhands-tools/openhands/tools/terminal/terminal/subprocess_terminal.py \
  tests/tools/terminal/test_process_priority.py \
  tests/tools/terminal/test_pool_integration.py
0 errors, 0 warnings, 0 informations

Pre-commit was run after each changed file. On this Windows host, its default
pyright invocation reports the existing POSIX-only pty, fcntl, and os.setsid
symbols as missing in subprocess_terminal.py; the explicit Linux-platform
pyright command above is clean. Ruff, pycodestyle, import rules, and tool
registration checks pass for all changed files.

I do not have macOS hardware in this environment. The Darwin argv path is covered
by the policy test, while the end-to-end taskpolicy -c utility measurement is
the one documented by the OpenHands member in the original product issue.

Video/Screenshots

Not applicable for a process-scheduling backend change; reproducible process-level
test output is included above.

Design Doc

Not included; the change is limited to one internal policy helper and four
existing process-spawn call sites.

Type

• Bug fix
• Feature
• Refactor
• Breaking change
• Docs / chore

Notes

• Scope is intentionally limited to the terminal tool. File editor helpers, MCP
  servers, and browser processes are unchanged.
• Linux resolves nice before applying the terminal's possibly restricted
  PATH; if the utility is unavailable, terminal startup remains functional.
• macOS uses the measured utility QoS class, not the much more restrictive
  background class.