Add Go backend contract test harness (api-contracts)

justfile

@@ -234,6 +234,14 @@ test *args=("-short ./..."):
 
     go test {{ args }} -covermode set -coverprofile=cover.out
 
+# Run API contract tests against the Go binary (requires `just build` first)
+test-contracts:
+    #!/usr/bin/env bash
+    set -eou pipefail
+    {{ _with_debug }}
+
+    cd test/api-contracts && npx vitest run
+
 # Execute Python script tests (licenses, prepare-release, etc.)
 test-scripts:
     #!/usr/bin/env bash

test/api-contracts/README.md

@@ -0,0 +1,54 @@
+# Publisher API Contract Tests
+
+Contract tests that validate the HTTP API surface of the Publisher backend. These ensure that the Go backend and a future TypeScript backend produce identical responses for the same API calls.
+
+## Architecture
+
+```
+Test code  →  Publisher binary (Go)
+               GET/POST/PUT/PATCH/DELETE /api/*
+```
+
+A single server is involved: the Publisher binary, which is spawned as a subprocess. Tests call Publisher's own REST API and assert on response status codes and body shapes.
+
+## What's tested
+
+- **Configurations** — CRUD operations on `.posit/publish/*.toml` files
+- **Credentials** — Create, list, delete server credentials
+- **Deployments** — CRUD operations on `.posit/publish/deployments/*.toml` files
+
+## Client implementations
+
+| Client | Description |
+|--------|-------------|
+| `GoHttpClient` | Calls Publisher's HTTP API (the current Go binary) |
+| `TypeScriptDirectClient` | Stub for future TS backend (all methods throw) |
+
+Set `API_BACKEND=typescript` to run against the TS client once implemented.
+
+## Running
+
+```bash
+# Build the Go binary first
+just build
+
+# Run contract tests
+just test-contracts
+
+# Or directly
+cd test/api-contracts && npx vitest run
+
+# Update snapshots
+cd test/api-contracts && npx vitest run --update
+```
+
+## Adding tests
+
+1. Add a method to the `BackendClient` interface in `src/client.ts`
+2. Implement it in both `src/clients/go-http-client.ts` and `src/clients/typescript-direct-client.ts`
+3. Create a test file in `src/endpoints/`
+4. Use `getClient()` from `src/helpers.ts` to get the appropriate client
+
+## Fixture workspace
+
+`src/fixtures/workspace/` contains a minimal project that Publisher needs to start. The workspace is copied to a temp directory for each test run.