Fix: convert 02_expand_context.py from notebook-style to valid Python script

[ShaileshRawat1988]

What this PR does

• Converts notebook/Markdown-style content in 02_expand_context.py into valid Python comments and docstrings
• Removes fenced code blocks and raw Markdown headings that caused syntax errors
• Ensures the file compiles and runs as a standalone Python script

Why this change is needed

The file currently contains Markdown prose and fenced code blocks that are not valid Python syntax, causing immediate SyntaxError during compilation or execution. This prevents users from running the guide locally as intended.

What this PR does NOT do

• Does not change instructional content or examples
• Does not refactor logic or APIs
• Does not introduce new providers or models

Verification

• python -m py_compile 10_guides_zero_to_hero/02_expand_context.py