Add FastCGI protocol support

[benoitc]

Summary

• Add FastCGI binary protocol support via --protocol fastcgi
• Enable gunicorn to receive requests from nginx using fastcgi_pass directive
• Include multiplexing support for concurrent requests on a single connection

Changes

New module: gunicorn/fastcgi/

• constants.py - Protocol constants (record types, roles, flags, status codes)
• errors.py - FastCGI-specific exceptions (InvalidFastCGIRecord, UnsupportedRole, ForbiddenFastCGIRequest)
• message.py - FastCGIRequest parser with multiplexing support (FastCGIConnectionState)
• parser.py - FastCGIParser extending base Parser
• response.py - FastCGIResponse with STDOUT/END_REQUEST record framing

Configuration

• --protocol fastcgi - Enable FastCGI protocol
• --fastcgi-allow-from - IP allowlist (default: 127.0.0.1,::1)

Integration

• gunicorn/http/__init__.py - Protocol routing for FastCGI
• gunicorn/http/wsgi.py - create_fastcgi() for WSGI environ from CGI vars
• gunicorn/workers/base.py - FastCGI error handling

Usage

gunicorn --protocol fastcgi -b 127.0.0.1:9000 myapp:app

nginx configuration:

upstream gunicorn {
    server 127.0.0.1:9000;
}
location / {
    fastcgi_pass gunicorn;
    include fastcgi_params;
}

Tests

• 60 unit tests in tests/test_fastcgi.py
• 24 Docker integration tests in tests/docker/fastcgi/
• All 565 tests pass (541 existing + 24 integration)

[benoitc]

converting as a draft. support for asgi need to be added