Breaking Change Detection for PRs

Adds .github/workflows/breaking-change-check.yml to analyze every PR that
touches src/bedrock_agentcore/ for public API breaking changes, and posts a
warning comment on the PR if any are found.

What griffe detects

griffe is a static analysis library for Python APIs used by mkdocstrings,
Rye, and the broader Python packaging ecosystem. It understands:

• Removed public modules, classes, functions, and attributes
• Removed or changed __all__ exports
• Changed function signatures: removed parameters, parameter kind changes
  (positional → keyword-only), default value changes
• Class hierarchy changes (removed base classes)
• Multi-line signatures, decorators, type annotations

Behavior

• ✅ No breaking changes → posts a green check comment
• ⚠️ Breaking changes found → posts a detailed list in markdown
• 🛑 griffe errors (e.g. transient git issue) → posts an info comment, job stays green
• Does not block merge — informational only