Add assert_json_snapshot function

[MatthewMckee4]

Summary

• Add assert_json_snapshot function that serializes values via json.dumps(value, sort_keys=True, indent=2) for deterministic, readable snapshot output of dicts and lists
• Extract shared assert_snapshot_impl helper from assert_snapshot to avoid duplicating snapshot comparison/write logic between the two functions
• Supports all existing snapshot features: inline snapshots, named snapshots, --snapshot-update, filters via snapshot_settings, and the pending/accept workflow
• Add type stubs, Python exports, and documentation

Test plan

• 6 new integration tests: basic JSON, nested data, update mode, inline, filters, non-serializable value
• All 580 existing tests pass (just test)
• prek run -a passes (clippy, fmt, ruff, etc.)

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing add-assert-json-snapshot (786e461) with main (9d43f84)}

⚠️ Unknown Walltime execution environment detected

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Summary

✅ 1 untouched benchmark