feat: Add system-prompt-file input for file-based system prompts

This enhancement adds the ability to load a system prompt from a file, similar to
the existing prompt-file functionality, providing more flexibility when working with
complex system prompts.

Key changes:
- Added new `system-prompt-file` input to action.yml with proper description
- Updated main.ts implementation to handle file-based system prompts with:
  - File existence checking and appropriate error handling
  - Proper precedence (system-prompt-file takes priority over system-prompt)
  - Consistent error messages with existing prompt-file implementation

Test coverage added:
- Basic usage: Test verifies system-prompt-file loads content correctly
- Error handling: Test ensures proper errors when system-prompt-file doesn't exist
- Precedence: Test confirms system-prompt-file overrides system-prompt when both exist
- Integration: Test validates both prompt-file and system-prompt-file work together
- Max tokens: Test verifies custom token limits are properly passed to the model
- Testing infrastructure: Improved mock implementations that detect unexpected calls

Documentation:
- Updated README.md with system-prompt-file in inputs table
- Added dedicated usage example for system-prompt-file
- Fixed formatting in inputs table

CI/CD:
- Updated workflow to test system-prompt-file functionality with actual file content

This feature maintains backward compatibility while adding a useful option
for workflows that need to use more complex system prompts stored in files.

dist/index.js

@@ -33574,7 +33574,22 @@ async function run() {
         else {
             throw new Error('prompt is not set');
         }
-        const systemPrompt = coreExports.getInput('system-prompt');
+        const systemPromptFile = coreExports.getInput('system-prompt-file');
+        const systemPromptString = coreExports.getInput('system-prompt');
+        let systemPrompt;
+        if (systemPromptFile !== undefined && systemPromptFile !== '') {
+            if (!fs.existsSync(systemPromptFile)) {
+                throw new Error(`System prompt file not found: ${systemPromptFile}`);
+            }
+            systemPrompt = fs.readFileSync(systemPromptFile, 'utf-8');
+        }
+        else if (systemPromptString !== undefined && systemPromptString !== '') {
+            systemPrompt = systemPromptString;
+        }
+        else {
+            // Use default system prompt
+            systemPrompt = 'You are a helpful assistant';
+        }
         const modelName = coreExports.getInput('model');
         const maxTokens = parseInt(coreExports.getInput('max-tokens'), 10);
         const token = coreExports.getInput('token') || process.env['GITHUB_TOKEN'];