Improve error message for unknown fields in semantic_model config

[theyostalservice]

Why

When a user adds an unsupported field inside the `semantic_model:` config object (e.g. `description:`, which the docs incorrectly showed as valid), dbt produces an unhelpful error:

```
at path ['semantic_model']: {'enabled': True, 'name': 'purchases', 'description': '...'} is not valid under any of the given schemas
```

The `anyOf` JSON Schema failure gives no indication of which field is wrong or what is allowed.

What

• `UnparsedSemanticModelConfig.validate()` — checks for unknown fields and raises `ValidationError` with a message naming the offending field(s) and listing the valid ones
• `UnparsedModelUpdate.validate()` — calls the nested validator before the full JSON Schema so the clear error fires first

After this change the error reads:

```
Unknown field(s) in semantic_model config: description. Valid fields are: config, enabled, group, name.
```

Tests:

• `tests/unit/utils/init.py`: adds `assert_fails_validation_with_message()` helper to `ContractTestCase` for asserting both that validation fails and that the error message is actionable — intended for reuse as we improve errors on other unparsed objects
• `tests/unit/contracts/graph/test_unparsed.py`: adds `TestUnparsedSemanticModelConfig` with four unit tests covering valid config, single extra field, message lists valid fields, and multiple extra fields

Refs

• Slack thread: C03KHQRQUBX / 1774638031.550019
• Related docs fix: Add semantic model parsing architecture doc (docs/arch/3.3_Semantic_Models.md) #12765

Drafted by claude-sonnet-4-6 under the direction of @theyostalservice

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.43%. Comparing base (eee9587) to head (1ddae80).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12766      +/-   ##
==========================================
+ Coverage   91.41%   91.43%   +0.01%     
==========================================
  Files         203      203              
  Lines       25844    25853       +9     
==========================================
+ Hits        23626    23639      +13     
+ Misses       2218     2214       -4     

Flag	Coverage Δ
integration	88.30% <33.33%> (+0.01%)	⬆️
unit	65.57% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	65.57% <100.00%> (+0.01%)	⬆️
Integration Tests	88.30% <33.33%> (+0.01%)	⬆️

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[QMalcolm]

LGTM! Thank you for doing this work!