Add tools to fetch pipeline templates

[mathislucka]

Overview

We want to add tools and mcp tools that allows our users to fetch a single pipeline and to list pipeline templates.
The tools will use the PipelineTemplateResource to either list pipeline templates or get a single template.

Context

• have a look at the tools in src/deepset_mcp/tools/pipeline.py to see how we define tools
• check out src/deepset_mcp/tools/formatting_utils.py to see how we serialize the Pipeline models to strings
• the PipelineTemplateResource is in src/deepset_mcp/api/pipeline_template/resource.py
• relevant models are in src/deepset_mcp/api/pipeline_template/models.py
• refer to src/deepset_mcp/main.py to see how we use tools with mcp

Requirements

• we have 2 tools (list_pipeline_templates, get_pipeline_template)
• both tools can also be used with MCP
• the tools follow the same structure as for the pipeline tools
• answers are serialized to strings (extract logic into separate helper functions)
• errors are also returned as strings
• the mcp tools are added to main.py

Testing

• all tests go into the test directory
• refer to test/unit/tools/test_pipeline.py to see how we test tools
• create ONLY unit tests for the pipeline template tools
• use a FakePipelineTemplateResource in the tests
• we do not test the mcp tools
• tests need full type annotations (including return types)