Implement ContentBuilder for Structured Content Generation

[butschster]

This PR introduces a ContentBuilder system that replaces string concatenation in source fetchers with a more structured, maintainable approach to content generation. The implementation follows the builder pattern to create a fluent API for building content blocks.

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Add custom tags to TextSource for better content identification

This PR introduces a new tag property to the TextSource class, allowing users to provide custom identifiers for text content blocks. The tag replaces the hardcoded "TEXT CONTENT" label in the generated output, making it easier to identify and distinguish between different sections of content.

Example

Before:

// TEXT CONTENT:
# API Documentation

This document provides an overview of the API.
// END OF TEXT CONTENT
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After (with custom tag):

// INTRO:
# API Documentation

This document provides an overview of the API.
// END OF INTRO
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