Make max buffer limit configurable via environment variable

[nandesh-chicory]

Summary

This PR adds support for configuring the maximum buffer size through the CLAUDE_MAX_BUFFER_SIZE environment variable, allowing users to adjust the buffer limit based on their specific needs without modifying the code.

Changes

• Modified src/claude_code_sdk/_internal/transport/subprocess_cli.py to make the buffer limit configurable
• Added _DEFAULT_MAX_BUFFER_SIZE constant to preserve the default 1MB limit
• Buffer size now reads from CLAUDE_MAX_BUFFER_SIZE environment variable with fallback to default

Benefits

• Users can now adjust buffer limits for large JSON messages without code changes
• Maintains backward compatibility - default behavior unchanged
• Simple configuration via environment variable

Usage

# Use default 1MB buffer
python your_script.py

# Use custom 5MB buffer
CLAUDE_MAX_BUFFER_SIZE=5242880 python your_script.py

Testing

• Existing tests should continue to pass as default behavior is unchanged
• The change is backward compatible