chore: Pass get params via `params` instead of encoding them by hand

[mathislucka]

First, check REPO.md to view our contribution guidelines.

Then implement the following changes:

Update the following resources to pass URL params as params to client._request instead of hand-rolling the URL:

• src/deepset_mcp/api/haystack_service/resource.py
• src/deepset_mcp/api/pipeline/resource.py
• src/deepset_mcp/api/pipeline_template/resource.py
• also check the corresponding tests to see if you need to change urls for the fakes we are using (test/unit/api as well as tool tests in test/unit/tools