fix(shell): prevent pager hangs in non-interactive mode

[cagataycali]

Description

Fixes the "WARNING: terminal is not fully functional" issue that causes the shell tool to hang indefinitely when running pager-based commands (like git diff, git log, man, etc.) in non-interactive mode.

Related Issues

Fixes #360

Root Cause

The CommandExecutor.execute_with_pty() method was not setting the TERM environment variable in the child process. When commands spawn pagers like less, the pager:

1. Detects incomplete terminal setup
2. Prints "WARNING: terminal is not fully functional"
3. Gets stuck waiting for user input that never comes

Solution

1. Set TERM environment variable: Sets TERM=xterm-256color if not already set, enabling proper terminal capability detection
2. Disable pagers in non-interactive mode: Sets GIT_PAGER, PAGER, and MANPAGER to cat to bypass pagers entirely when running non-interactively

Changes

• src/strands_tools/shell.py: Add environment setup in child process before executing command
• tests/test_shell.py: Add test to verify the fix is in place

Testing

• All 32 existing tests pass
• Added new test test_command_executor_sets_pager_env_in_non_interactive
• Manual testing confirms git commands no longer hang

# Before fix - HANGS
git log --oneline -3  # ❌ Times out

# After fix - WORKS
git log --oneline -3  # ✅ Returns immediately

Checklist

• I have read the CONTRIBUTING document
• I have added tests that prove my fix is effective
• I have run ruff check and ruff format
• All existing tests pass
• My changes generate no new warnings

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)