docs: add README for examples/mcp_demo

[LeonSGP43]

Description

Adds examples/mcp_demo/README.md in the same style as examples/langchain_demo/README.md. examples/mcp_demo contains three runnable demo scripts plus mock-server generators, but no README — the demo is undiscoverable and its entry points (module invocation with PYTHONPATH=., the OPENAI_API_KEY requirement for the eval) are only documented inside script docstrings.

Type of Change

• Documentation update

Changes Made

• Added examples/mcp_demo/README.md: overview, prerequisites, how to run each demo script (including PYTHONPATH=. module invocation), mock-server generation, and expected output for the eval demo (OPENAI_API_KEY requirement noted).

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

# Docs-only change: no Python surface touched, so pytest/ruff/mypy are unaffected.
# Manual checks performed:
#   - every command in the README cross-checked against the scripts' argparse/docstrings
#   - structure and tone match examples/langchain_demo/README.md
#   - referenced file paths all exist in examples/mcp_demo/

Real Behavior Proof

• Environment: local checkout of main + branch
• Exact command / steps: walked each documented command against the actual scripts in examples/mcp_demo/; verified every referenced path exists
• Observed result: documented invocations match the scripts' __main__ entry points and flags; env-var requirements match the code
• Not tested: live OpenAI-backed eval run (needs OPENAI_API_KEY); mock-server demos were verified by reading the code paths

Runtime Rollout Safety

• Rollout-managed feature(s): none — example documentation only
• Minimum rollout channel: N/A
• Stable/default behavior changed: none
• Kill switch / disable path: N/A
• Unsafe override required: none
• Qualification impact: none
• Rollback path: revert the single-file commit

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I did not edit CHANGELOG.md — it is generated by release-please from my Conventional Commit PR title

Additional Notes

Docs-only PR: pytest/ruff/mypy items intentionally unchecked (no Python code changed).

[github-actions]

PR governance

This PR follows the template and is marked ready for human review.