fix(openai): bound native shell output

[jdchawla29]

Summary

• enforce the OpenAI shell call's effective max_output_length for each command
• apply the same bounded stdout/stderr data to structured output and display content
• preserve deterministic output prefixes and suffixes with an explicit truncation marker
• reject malformed or non-positive limits before executing commands
• cap missing or oversized limits at the Responses API's 10 MiB text-field ceiling

Root cause

The native OpenAI shell adapter returned max_output_length as metadata but copied the complete command result into display content and structured stdout/stderr. No execution-boundary truncation occurred, so noisy output could exceed the Responses API field limit and terminate the rollout before grading.

Validation

• uv run --no-sync pytest -q hud/agents/openai/tools/tests hud/agents/tests/test_provider_native_tools.py hud/agents/tests/test_openai_agent.py hud/agents/tests/test_tool_agent.py (69 passed)
• Ruff format and lint checks
• Pyright on the touched implementation and tests

Linear: HUD-2204

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Note

Medium Risk
Changes agent tool execution and result shaping on a critical path; behavior is well-tested but affects all native OpenAI shell calls.

Overview
OpenAI native shell now honors max_output_length instead of echoing full command output while only storing the limit as metadata—fixing rollouts that blew past Responses API text limits on noisy commands.

Execution parses and validates max_output_length (integer ≥ minimum, capped at 10 MiB); bad values return an error without running commands. Each command runs via conn.run with separate stdout/stderr, then _bound_output trims combined length per command using a truncation marker while keeping head/tail. Structured output and MCP text content both use the same bounded streams (one text block per command).

Tests cover huge stderr truncation, per-command limits, invalid limits, and default/ceiling effective limits; the fake SSH client now supports stderr.

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