fix(contracts): make DataContractRead.model_validate accept ORM rows (#455)

[larsgeorge-db]

Summary

Closes #455 — POST /api/data-contracts/{id}/clone returned HTTP 400 ("Invalid contract data") even though the cloned version was successfully written to the database. The bug was in response serialization, not the clone logic itself.

Root cause is a two-layer Pydantic v2 incompatibility:

1. The clone/history/personal-drafts routes call DataContractRead.model_validate(orm_row).model_dump() without from_attributes=True. Pydantic v2 does not auto-propagate the parent model's ORM-mode config into nested model validation when the parent is validated via model_validate(obj) (only when the kwarg is passed at the call site). The nested ServerConfig therefore tried to coerce a DataContractServerDb ORM row as a dict and raised Input should be a valid dictionary or instance of ServerConfig → caught by the generic except ValueError as e → 400.
2. Even with from_attributes=True propagating in, ServerConfig.properties (typed Dict[str, Any]) cannot validate the SQLAlchemy InstrumentedList of DataContractServerPropertyDb rows that the relationship returns — those are key/value rows, not a dict. This was a latent second-layer bug that fix Cannot Create a New Domain if Choosing a Parent One #1 exposed.

Changes

• routes/data_contracts_routes.py: pass from_attributes=True to all four DataContractRead.model_validate(...) call sites (clone, version history's current/parent/children/siblings, personal drafts).
• models/data_contracts_api.py:
  • Add from_attributes = True to ServerConfig.Config so future callers don't have to remember the kwarg.
  • Add a @field_validator('properties', mode='before') that flattens a list of {key, value} rows or dicts into the expected Dict[str, Any]. Plain-dict input is passed through unchanged.

Tests

src/backend/src/tests/unit/test_data_contracts_api_models.py (+184 lines):

• DataContractRead.model_validate(orm_row, from_attributes=True) now round-trips populated servers, team, roles, support, pricing, and sla_properties collections.
• ServerConfig.properties coercion exercised directly for object-rows (.key/.value), plain dict, and list-of-{key, value} dicts.

```
======================= 9 passed, 112 warnings in 0.32s ========================
```

All 42 tests across test_data_contracts_api_models.py, test_version_visibility.py, and test_version_family.py continue to pass.

Stacking

This PR is stacked on PR #448 (PRD #442 phase 3, branch feat-version-family-picker). Once #448 merges to main, please retarget this PR's base to main.

Test plan

• Unit tests pass (pytest src/tests/unit/test_data_contracts_api_models.py)
• Manual smoke: clone a contract via UI on the version-family worktree and confirm the response body now contains the cloned id/version and populated servers instead of 400.
• Manual smoke: open the version history dialog for a contract with servers and confirm current/parent/children/siblings all serialize cleanly.