docs:Improve the contributing.md for testing purposes

[fede-dash]

Fix test setup documentation - missing OpenAPI spec instructions

Problem

The current contributing documentation has a broken workflow that leaves developers stuck:

# Current docs say to run this, but openapi.yml doesn't exist:
npx prism mock path/to/your/openapi.yml

Issues:

• ❌ References openapi.yml file that doesn't exist in the repo
• ❌ Provides vague placeholder path path/to/your/openapi.yml
• ❌ Missing critical setup steps to obtain the OpenAPI specification
• ❌ Developers get "file not found" errors when following instructions

Solution

Add complete, executable instructions that actually work:

Before (broken):

# you will need npm installed
$ npx prism mock path/to/your/openapi.yml

After (fixed):

# 1. Download OpenAPI spec from .stats.yml (use latest valid openapi_spec_url)
curl -o openapi.yml [URL_FROM_STATS_YML]

# 2. Copy to main project directory
cp openapi.yml path/to/anthropic-sdk-python/

# 3. Start mock server
npx prism mock openapi.yml

# 4. Run tests
./scripts/test

Changes Made

• ✅ Added missing download step for OpenAPI specification
• ✅ Provided concrete commands instead of placeholder paths
• ✅ Made workflow executable from start to finish
• ✅ Added context explaining prerequisites and troubleshooting

Testing

• Verified .stats.yml contains openapi_spec_url
• Tested download and setup process
• Confirmed mock server starts successfully
• Validated tests run against mock server

Impact

• New contributors can now follow the documentation without getting stuck
• Reduces support burden from "documentation doesn't work" issues
• Improves developer experience with clear, step-by-step guidance

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Type: Documentation fix
Breaking: No
Fixes: Missing OpenAPI spec setup instructions