feat(dev): add showcase:generate command (#9347)

Author: Hectorhammett

Gax/README.md

@@ -80,6 +80,8 @@ be found for Mac or Windows.
     > vendor/bin/phpunit
     ```
 
+    For Showcase Conformance testing and client generation, see [tests/Conformance/README.md](tests/Conformance/README.md).
+
 4.  Run static analysis tools
 
     ```sh

Gax/tests/Conformance/README.md

@@ -0,0 +1,46 @@
+# GAX Showcase Conformance Testing
+
+This directory contains the integration conformance test suite (`ShowcaseTest.php`) and generated client SDK for `gapic-showcase`.
+
+## Prerequisites
+
+- **Protobuf Compiler (`protoc`)**: Ensure `protoc` is installed and available in your `$PATH`.
+- **Dev Dependencies**: Ensure dev dependencies are installed:
+  ```sh
+  composer install -d dev/
+  ```
+
+## Generating the Showcase Client
+
+To generate or update the Showcase client SDK, message classes, and metadata using the PHP CLI tool:
+
+```sh
+# From the repository root, run the console command:
+./dev/google-cloud showcase:generate
+```
+
+Options:
+- `-o, --out-dir <dir>`: Output directory relative to repository root (defaults to `Gax`).
+- `-p, --showcase-path <dir>`: Path to local `gapic-showcase` repository (defaults to installed vendor dependency).
+- `-g, --generator-path <dir>`: Path to local `gapic-generator-php` (defaults to installed vendor dependency).
+- `-a, --googleapis-path <dir>`: Path to `googleapis` repository (defaults to submodule in `gapic-generator-php`).
+
+## Running Conformance Tests
+
+1. Install and launch the matching `gapic-showcase` mock server in the background:
+   ```sh
+   # Option 1 (curl)
+   curl -L https://github.com/googleapis/gapic-showcase/releases/download/v0.41.1/gapic-showcase-0.41.1-linux-amd64.tar.gz | tar -zx
+   ./gapic-showcase run &
+
+   # Option 2 (go install)
+   go install github.com/googleapis/gapic-showcase/cmd/gapic-showcase@latest
+   gapic-showcase run &
+   ```
+
+2. Run the PHPUnit conformance test suite:
+   ```sh
+   vendor/bin/phpunit -c Gax/phpunit-conformance.xml.dist
+   # or from the Gax directory:
+   cd Gax && vendor/bin/phpunit tests/Conformance/ShowcaseTest.php
+   ```