Python(feat): pytest summary output

[alexluck-sift]

• New end-of-run Sift report panel: report name in the section title, then Status, Steps, Measurements (color-coded: green pass, red fail/error, yellow skip), Test case, System, Log file, and a clickable Report link (online) or upload command (offline). Suppressed under -q.
• New session-start header showing the SDK version and active mode (e.g. Sift: sift-stack-py 0.17.0, online mode).
• New --sift-open-report / sift_open_report flag opens the report in a browser at session end (gated to interactive terminals so it does nothing in CI or captured shells).
• New --sift-report-url-base / sift_report_url_base / SIFT_APP_URL env var for on-prem and custom deployments. Online auto-derives the web-app URL for known Sift hosts; the override fills the gap for others. SiftClient.app_url exposes the resolved origin so non-plugin consumers can use it too.
• Richer step.fail_if_measurements_failed() output: lists each out-of-bounds measurement by name, value, and bounds (e.g. [FAILED] b = 99.0 (min 0.0, max 2.0)). Backed by TestMeasurement.__str__, so the same format is available in logs and the REPL.
• ReportContext now retains created steps and measurements with step_status_counts / measurement_counts properties. Docs and unit/integration tests updated for the new panel, options, and failure output.

Verification

Sift info added to pytest header:

fail_if_measurements_failed() output in pytest:

Offline summary:

Online summary:

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-594/

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