Add --diff to compare a response against a JSON baseline

[jongio]

What

Adds a --diff <file> flag that compares the JSON response against a saved baseline and prints a unified diff, exiting non-zero when they differ. This turns azd rest into a snapshot-testing tool for APIs in CI.

Both the response and the baseline are canonicalized (parsed as JSON, re-encoded with sorted object keys and normalized whitespace) before comparison, so a change in key order or formatting does not produce a false diff. Only real value changes show up.

Behavior

• Match: prints nothing, exits 0.
• Drift: prints a unified diff (baseline vs response, 3 lines of context) to stdout and exits 1.
• Missing or unreadable baseline, a non-JSON baseline, or a non-JSON response: exits 2 (usage/config), consistent with the existing exit-code contract.

--diff is terminal: it replaces the normal body output so the diff is the only thing on stdout, which keeps it easy to capture or pipe.

Example

# Save a known-good response once
azd rest get https://management.azure.com/subscriptions/{sub}/resourceGroups/{rg}?api-version=2021-04-01 > baseline.json

# Later, in CI, fail the job if the response drifts
azd rest get https://management.azure.com/subscriptions/{sub}/resourceGroups/{rg}?api-version=2021-04-01 --diff baseline.json

Notes

• Reuses client.IsJSON for the response-type guard, matching how service.go and color.go already gate JSON handling.
• Promotes github.com/pmezard/go-difflib from an indirect to a direct dependency (it was already in the module graph via testify). No new third-party code is pulled in.
• Docs updated: CLI reference flag table, exit-code table, and a README example.

Tests cover key-order insensitivity (top level and nested), drift output, and the three exit-code-2 cases, plus end-to-end match and drift through Execute.

Closes #266

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Build Info:

• Version: 0.5.0-pr273
• Commit: afd5829
• Release: View pre-release