Add .github/copilot-instructions.md for AI-assisted development

.github/copilot-instructions.md

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+# Copilot Instructions for sonic-restapi
+
+## Project Overview
+
+sonic-restapi provides a RESTful API server for SONiC switches, exposing HTTPS endpoints for dynamic network configuration. The server (`go-server-server`) allows external systems to configure switch features like VxLAN tunnels, routes, and ARP entries programmatically.
+
+## Architecture
+
+```
+sonic-restapi/
+├── go-server-server/       # Go REST API server implementation
+├── arp_responder/          # ARP responder module
+├── arpthrift/              # ARP Thrift interface
+├── arpthrifttest/          # ARP Thrift tests
+├── Makefile                # Build system
+├── Dockerfile              # Production Docker image
+├── Dockerfile.build        # Build environment Docker image
+├── Dockerfile.test         # Test Docker image
+├── Dockerfile.*.prod       # Platform-specific production images
+├── build.sh                # Build script
+├── CreateMockPort.sh       # Test helper script
+├── azurepipeline.yml       # CI pipeline
+└── README.md
+```
+
+### Key Concepts
+- **HTTPS REST API**: Exposes configuration endpoints over HTTPS
+- **Redis backend**: Reads from and writes to SONiC Redis databases
+- **Docker deployment**: Runs as a Docker container on SONiC switches
+- **go-server-server**: The main Go binary implementing REST endpoints
+
+## Language & Style
+
+- **Primary languages**: Go (server), Python (tests), Shell (scripts)
+- **Go conventions**: Standard Go formatting (`gofmt`), idiomatic error handling
+- **Indentation**: Go: tabs (gofmt default), Python: 4 spaces
+- **API design**: RESTful conventions — proper HTTP methods, status codes, JSON payloads
+- **Naming**: Go exported names in `PascalCase`, unexported in `camelCase`
+
+## Build Instructions
+
+```bash
+# Build using Docker
+./build.sh
+
+# This generates two Docker images:
+# - rest-api-image: Production deployment
+# - rest-api-image-test_local: Local testing with embedded Redis
+
+# The production image is also saved as rest-api-image.gz
+```
+
+## Testing
+
+```bash
+# Run container locally with embedded Redis
+docker run -d --rm -p8090:8090 -p6379:6379 \
+  --name rest-api --cap-add NET_ADMIN --privileged \
+  -t rest-api-image-test_local:latest
+
+# Run tests
+cd test
+pytest -v
+
+# Check logs
+docker exec -it rest-api bash
+cat /tmp/rest-api.err.log
+```
+
+### Deploying on a Switch
+```bash
+# Copy rest-api-image.gz to the switch
+docker load < rest-api-image.gz
+docker run -d -p=8090:8090/tcp \
+  -v /var/run/redis/redis.sock:/var/run/redis/redis.sock \
+  --name rest-api --cap-add NET_ADMIN --privileged \
+  -t rest-api-image:latest
+```
+
+## PR Guidelines
+
+- **Signed-off-by**: Required on all commits
+- **CLA**: Sign Linux Foundation EasyCLA
+- **Testing**: Include API tests for new endpoints
+- **Security**: HTTPS endpoints must maintain proper authentication and authorization
+- **CI**: Azure pipeline checks must pass
+
+## Gotchas
+
+- **TLS certificates**: HTTPS requires proper certificate configuration
+- **Redis socket**: Production deployment mounts the SONiC Redis Unix socket
+- **Network namespace**: Container needs `NET_ADMIN` capability
+- **API versioning**: Maintain backward compatibility for existing API consumers
+- **Concurrent access**: Handle concurrent Redis operations safely
+- **Error responses**: Return proper HTTP status codes and error messages