Add comprehensive testing infrastructure

This commit adds a complete testing setup for the prettier_action:

- Adds BATS (Bash Automated Testing System) testing framework
- Creates unit tests for _git_setup() and _git_changed() functions
- Creates plugin validation tests to ensure proper prettier plugin format
- Creates integration tests for end-to-end workflows
- Adds automated test runner script (tests/run_tests.sh)
- Adds GitHub Actions workflow for CI/CD testing
- Includes ShellCheck linting for bash scripts
- Updates README with comprehensive testing documentation
- Updates .gitignore to exclude test artifacts

Test coverage includes:
- Git configuration with different identity modes
- File change detection
- Plugin name validation (official, community, and scoped formats)
- Working directory handling
- node_modules cleanup
- package-lock.json restoration
- only_changed file filtering
- Dry run behavior

The test suite can be run locally with ./tests/run_tests.sh and runs
automatically on all pushes and pull requests.

tests/plugin_validation_tests.bats

@@ -0,0 +1,99 @@
+#!/usr/bin/env bats
+
+# Integration tests for prettier plugin validation
+
+load 'test_helper'
+
+setup() {
+    setup_test_repo
+    mock_github_env
+    set_default_inputs
+}
+
+teardown() {
+    teardown_test_repo
+}
+
+# Test valid prettier plugin patterns
+@test "Valid @prettier/plugin-* format should pass validation" {
+    # Test the regex pattern used in entrypoint.sh
+    plugin="@prettier/plugin-php"
+
+    run bash -c "echo '$plugin' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+
+    [ "$status" -eq 0 ]
+}
+
+@test "Valid prettier-plugin-* format should pass validation" {
+    plugin="prettier-plugin-java"
+
+    run bash -c "echo '$plugin' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+
+    [ "$status" -eq 0 ]
+}
+
+@test "Valid @scope/prettier-plugin-* format should pass validation" {
+    plugin="@company/prettier-plugin-custom"
+
+    run bash -c "echo '$plugin' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+
+    [ "$status" -eq 0 ]
+}
+
+@test "Invalid plugin name should fail validation" {
+    plugin="some-random-package"
+
+    run bash -c "echo '$plugin' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+
+    [ "$status" -eq 1 ]
+}
+
+@test "Invalid plugin with wrong prefix should fail validation" {
+    plugin="@other/plugin-something"
+
+    run bash -c "echo '$plugin' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+
+    [ "$status" -eq 1 ]
+}
+
+@test "Multiple valid plugins should all pass validation" {
+    plugins="@prettier/plugin-php prettier-plugin-java @scope/prettier-plugin-custom"
+
+    for plugin in $plugins; do
+        run bash -c "echo '$plugin' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+        [ "$status" -eq 0 ]
+    done
+}
+
+@test "Plugin name with uppercase should fail validation" {
+    plugin="@prettier/plugin-PHP"
+
+    run bash -c "echo '$plugin' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+
+    [ "$status" -eq 1 ]
+}
+
+@test "Plugin name with numbers or underscores should fail validation" {
+    plugin1="@prettier/plugin-test123"
+    plugin2="prettier-plugin-test_name"
+
+    run bash -c "echo '$plugin1' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+    [ "$status" -eq 1 ]
+
+    run bash -c "echo '$plugin2' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+    [ "$status" -eq 1 ]
+}
+
+@test "Official prettier plugins should be recognized" {
+    # List of known official prettier plugins
+    plugins=(
+        "@prettier/plugin-php"
+        "@prettier/plugin-ruby"
+        "@prettier/plugin-xml"
+    )
+
+    for plugin in "${plugins[@]}"; do
+        run bash -c "echo '$plugin' | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'"
+        [ "$status" -eq 0 ]
+    done
+}