docs: add encapsulation boundary rules (Principle 7)

[colombod]

Summary

Adds a new principle to PYTHON_BEST_PRACTICES.md addressing the critical anti-pattern of accessing private attributes via getattr(obj, '_internal', ...).

Motivation

This PR was motivated by microsoft/amplifier-app-cli#70 which demonstrates a fix that reaches into private internal state instead of using public APIs:

# The anti-pattern from PR #70
bundle_resolver = getattr(resolver, "_bundle", resolver)  # Unwrap layer 1
activator = getattr(bundle_resolver, "_activator", None)   # Unwrap layer 2

The comment "Pattern from session_spawner.py" shows this anti-pattern is spreading across the codebase.

What's Added

New Principle 7: Respect Encapsulation Boundaries covering:

• Why accessing private state is dangerous (changes without warning, multiplied fragility)
• Table of anti-patterns and correct approaches
• Real code example from PR #70
• Decision framework: when tempted to access _private, what to do instead
• Litmus test for detecting the problem

Related

• Companion PR for amplifier-core: adds similar guidance to DESIGN_PHILOSOPHY.md
• Motivating issue: fix: unwrap AppModuleResolver to find activator in self-healing amplifier-app-cli#70

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🤖 Generated with Amplifier

Co-Authored-By: Amplifier 240397093+microsoft-amplifier@users.noreply.github.com

[colombod]

Closing - the foundation PR (microsoft/amplifier-foundation#42) covers the general principle with language-agnostic examples, which is sufficient.