Improve flower shop happy path logs with cart snapshots

[n-watts]

What changed

• Added a compact cart snapshot helper to the flower_shop happy path client.
• Logs item titles + quantities and the current total after key checkout session updates:
  • session creation
  • adding items
  • applying discount

Why

• Makes the sample output easier to follow and debug (you can see what’s in the cart at each step), without logging any PII or payment details.

Test

• Ran the python server + flower_shop simple_happy_path_client.py end-to-end locally.

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[0x8000-0000]

Please check in the changed rest/python/client/flower_shop/sample_output/happy_path_dialog.md as well.

Ensure that there are no linter or formatting reports (we are adding linter actions to GitHub).

[n-watts]

Fixed remaining ruff docstring issues (imperative mood + whitespace).
ruff check . and ruff format --check . now pass cleanly (run from rest/python/client/flower_shop).

sample_output/happy_path_dialog.md remains unchanged since this PR only affects runtime logging.