feat: add agent decorators for configuration-as-code

[Cocossoul]

Description

Adds an agent_decorators module to the SAP Cloud SDK for Python. This module provides three decorator functions — @prompt_section, @agent_config, and @agent_model — that developers apply to zero-argument functions to expose agent configuration fields (prompts, settings, model selections) to external low-code tooling. The decorated functions remain fully callable and return their coded defaults unchanged; the decorators act as static markers that tooling discovers and extracts at build or deploy time.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

How to Test

1. Install the SDK and import the new module:

   from sap_cloud_sdk.agent_decorators import prompt_section, agent_config, agent_model

2. Verify decorated functions remain callable and return their default values:

   @prompt_section(key="prompts.system", label="System Prompt", description="Main prompt")
   def system_prompt() -> str:
       return "You are a helpful assistant."
   
   assert system_prompt() == "You are a helpful assistant."

3. Verify key validation raises AgentDecoratorError for empty or whitespace-only keys:

   from sap_cloud_sdk.agent_decorators.exceptions import AgentDecoratorError
   import pytest
   
   with pytest.raises(AgentDecoratorError):
       @prompt_section(key="", label="L", description="D")
       def bad(): return ""

4. Run the unit test suite:

   uv run pytest tests/agent_decorators/ -q

Checklist

• My code follows the project's Code Guidelines
• I have added type hints for all public APIs
• I have added unit tests covering the new functionality
• I have added or updated the user guide (user-guide.md) in the module directory
• All existing tests pass locally
• My commits follow Conventional Commits format
• No confidential or SAP-internal information is included

Disclaimer: Do not include SAP-internal or customer-specific information in this PR. This is a public repository.

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