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.

action.yml

@@ -29,6 +29,10 @@ inputs:
     description: The system prompt for the model
     required: false
     default: 'You are a helpful assistant'
+  system-prompt-file:
+    description: Path to a file containing the system prompt
+    required: false
+    default: ''
   max-tokens:
     description: The maximum number of tokens to generate
     required: false