Add draft, attachment, and RAG example scripts

[josephfeleke]

Summary

• Three new runnable examples under examples/:
  • draft_workflow.py — create → update → send a draft (covers the three draft-tool verbs).
  • attachment_demo.py — outbound base64 SendAttachmentSpec and inbound presigned-URL download.
  • rag_inbox.py — keyword search via AgentMailRetriever, plus an opt-in semantic path that pipes AgentMailLoader().load() into InMemoryVectorStore + OpenAI (only runs when OPENAI_API_KEY is set).
• Refactor examples/send_one_email.py to split env wiring (main()) from the actual send call (run(client, ...)), matching the structure used by the new examples and making it unit-testable.
• New tests/unit_tests/test_examples.py (7 tests) drives each example's run()/helper against a mocked SDK — no network, no real keys, no LLM calls.

Test plan

• ruff check . — clean.
• pytest — 35 passed (28 existing + 7 new) in ~0.6s.
• Each example module imports cleanly under python -c "import <example>".
• Reviewer spot-checks the docstring env-var lists against the helpers each example calls.

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Summary by cubic

Add three runnable examples for drafts, attachments, and inbox RAG search, and refactor send_one_email for consistency and testability. Includes unit tests that mock the SDK to run everything without network calls or real keys.

• New Features

  • examples/draft_workflow.py: create → update → send a draft.
  • examples/attachment_demo.py: send base64 attachment via SendAttachmentSpec and resolve a presigned download URL.
  • examples/rag_inbox.py: keyword search with AgentMailRetriever; optional semantic path using AgentMailLoader + in-memory vectors with OpenAI when OPENAI_API_KEY is set.

• Bug Fixes

  • CI: configure setup-uv@v3 to cache from pyproject.toml to restore caching and unblock the matrix.

^{Written for commit 4ca19ee. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

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