feat: rewrite middleware notebooks for Responses API coverage (#171)

* feat: rewrite middleware notebooks for Responses API coverage and fix ConversationMemory bug

Rewrite all 4 middleware notebooks to demonstrate both Chat Completions and
Responses API modes side-by-side. Each notebook now shows a two-model setup,
uses format_content()/inspect_response() helpers to display readable text for
both string and list-of-blocks content, and verifies provider ID stripping on
cached content.

Fix ConversationMemoryMiddleware failing to store Responses API messages due to
SemanticMessageHistory requiring string content. Added _content_to_str() helper
to normalize list-of-blocks content to plain strings before storage.

New integration tests verify the fix with real Redis (no mocks).

* fix: correct content block type check in test_memory_responses_api_recall

The test checked for block type "output_text" but Responses API content
blocks use type "text". Changed to check for "text" key presence instead,
matching the format_content() pattern used elsewhere.

Author: Brian Sam-Bodden