[BUG] Bash tool unexpectedly runs all commands in background mode on macOS CLI, output never returned

Environment:

• macOS 15 Sequoia (Darwin 24.6.0)
• Claude Code CLI v2.1.114
• Model: claude-opus-4-7

Describe the bug:
All Bash tool calls return a background task ID instead of executing inline and returning output. This happens without setting run_in_background: true, and persists even when using dangerouslyDisableSandbox: true.

Response from every Bash call:

Command running in background with ID: bxxxxxxxx. Output is being written to: /private/tmp/claude-501/<project-path>/tasks/bxxxxxxxx.output

Steps to reproduce:

1. Open Claude Code CLI on macOS
2. Ask Claude to run any Bash command (e.g. git status, date, ls)
3. All commands return a background task ID — output is written to /private/tmp/ but never surfaced back to the model

Expected behavior:
Bash commands execute inline and return output directly to the model.

Actual behavior:
All commands run as background tasks. The output file is written to disk but the model never receives the content, causing a complete loss of Bash tool functionality.

Additional context:

• Commands DO execute successfully (git commits/pushes ran correctly)
• Only Bash is affected — Read, Write, Edit, Glob, Grep, MCP tools all work normally
• Related to #19663 but different symptom (background task ID vs "(No content)")
• Workaround: user manually runs commands in terminal