feat: add Flask microservice example

[MarceloAdan73]

Description

Adds examples/flask_microservice.py — a multi-provider LLM API built with
Flask and llm_bridge. Includes:

• POST /chat — generic multi-provider chat (mock, callable, openai)
• POST /summarize — structured summarisation with prompt engineering
• GET /health — readiness check
• Callable provider — local text analyser (word count, language detection,
  reading time) with zero network dependencies

This is the first example showing llm_bridge integrated in a web framework,
demonstrating real-world patterns like per-request provider selection and
structured prompt templates.

Checklist

• ruff check . and black --check . pass
• No secrets, API keys, internal URLs, or proprietary content are included

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[MarceloAdan73]

I have read the CLA Document and I hereby sign the CLA

[opensource-SantanderAI]

Thanks for this — it's a genuinely useful addition and the first example that integrates llm_bridge into a web framework. The per-request provider selection and the zero-network callable text analyser are nice touches. ruff and black are clean.

One security change is required before we can merge, plus a couple of minor cleanups:

1. Do not ship debug=True + host="0.0.0.0" (in main()). The Werkzeug debugger enables arbitrary code execution if the port is reachable, and 0.0.0.0 exposes it on all interfaces — an insecure default for an example that people copy-paste (CodeQL/bandit flag this as py/flask-debug / B201). Please default to:

   app.run(host="127.0.0.1", port=5000, debug=False)

   If you want debug to remain toggleable, gate it behind an env var, e.g. debug=os.environ.get("FLASK_DEBUG") == "1".
2. Declare the example's dependency: the module needs Flask. The docstring mentions pip install flask, which is fine, but consider an examples/requirements.txt (or an [examples] extra) so it's reproducible.
3. Minor: the PR description has some leftover tutorial text ("Luego clic en Create pull request.") — please remove it.

With the debug/host fix the example is good to go. Thanks!