[GulrezQayyum]

Summary

Related to #19.

Adds offline contract tests for the existing VLLMModel and LiteLLMModel wrappers.

Coverage

• default vLLM model name and endpoint;
• custom vLLM model name and port;
• default LiteLLM model name and endpoint;
• custom LiteLLM model name and port.

The tests do not require a running vLLM or LiteLLM server, credentials, or network access.

Documentation

Updates the model integration documentation with:

• default local endpoints;
• custom model and port examples;
• clarification that wrapper construction does not contact a server;
• distinction between offline contract tests and live provider testing.

Verification

• uv run pytest -q — 125 passed
• uv run ruff check src/dlens/agents/_models.py tests/test_models.py
• git diff --check

Collaboration

The network-request construction test is covered separately in PR #23 to avoid overlapping changes.

This PR focuses on provider configuration and documentation, while #23 verifies that constructing the wrappers does not send HTTP requests.