docs: add interface boundary examples to implementation philosophy

[colombod]

Summary

Adds good/bad examples to IMPLEMENTATION_PHILOSOPHY.md showing why bypassing public interfaces creates fragile code.

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
bundle_resolver = getattr(resolver, "_bundle", resolver)
activator = getattr(bundle_resolver, "_activator", None)

What's Added

Two new practical examples in the "Practical Examples" section:

• Bad Example: Bypassing Public Interfaces - shows the anti-pattern of unwrapping internal state
• Good Example: Respecting Interface Boundaries - shows the correct approach

Plus the key rule: If you can't do something through the public interface, the fix is to extend the interface—not bypass it.

Why IMPLEMENTATION_PHILOSOPHY.md

This is development-time guidance that belongs where developers look when making implementation decisions. The "Practical Examples" section already establishes the pattern of showing good vs bad approaches with code.

Related

• Companion PR: docs: add encapsulation boundary rules (Principle 7) amplifier-bundle-python-dev#5 (Python-specific details)
• Motivating issue: fix: unwrap AppModuleResolver to find activator in self-healing amplifier-app-cli#70

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