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
  Luego clic en Create pull request.

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[MarceloAdan73]

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