feat(templates): add tojson_safe Jinja2 filter (closes #580)

[masukai]

Summary

Closes #580. Add a tojson_safe Jinja2 filter that tolerates datetime / date / time / Decimal / UUID — types that the standard tojson rejects with Object of type datetime is not JSON serializable. Unblocks BigQuery TIMESTAMP and Postgres numeric / uuid columns flowing through REST API body_template rendering without CAST(... AS STRING) workarounds in model SQL.

• tojson_safe calls json.dumps(value, default=_json_default, ensure_ascii=False) with a small whitelist:
  • datetime / date / time → ISO 8601 (obj.isoformat())
  • Decimal / UUID → str(obj)
  • anything else → TypeError, matching json.dumps
• Registered on both drt.templates.renderer and drt.destinations.staged_upload's local Jinja2 envs.
• The default tojson filter is unchanged — strictly opt-in, no behaviour change for existing templates. A regression test (test_tojson_strict_still_fails_on_datetime) locks this in.

Files

• drt/templates/renderer.py — _json_default, tojson_safe, _build_env() helper
• drt/destinations/staged_upload.py — import + register in _render
• tests/unit/test_renderer.py — 7 new tests
• docs/connectors/rest-api.md — new "Serializing datetime / Decimal / UUID columns" section
• CHANGELOG.md — [Unreleased] entry

Usage

body_template: |
  {
    "name":     {{ row.name | tojson_safe }},
    "metadata": {{ row | tojson_safe }}
  }

Test plan

• pytest tests/unit/test_renderer.py — 9 tests pass (2 existing + 7 new)
• pytest full suite — 1254 passed, 1 skipped
• ruff check on touched files — clean
• mypy on touched files — clean
• make check-i18n — clean
• Non-breaking guard: existing tojson still rejects datetime

🤖 Generated with Claude Code

[codecov]

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