fix: accept dict checksum on ManifestV12 unit test nodes (#94)

dbt 2.0 preview emits `unit_tests[].checksum` as a `Checksum` object
(`{"name": "", "checksum": ""}`) instead of a plain string. This broke
`dbt_ingestion` for tenants using dbt 2.0 (e.g., docusign integration 21).

Update `Disabled13.checksum` and `UnitTests.checksum` from
`Optional[str]` to `Optional[Union[str, Checksum]]` to accept both
the old string format and the new dict format.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: Sourabhchrs93

src/vendor/dbt_artifacts_parser/parsers/manifest/manifest_v12.py

@@ -4109,7 +4109,7 @@ class Disabled13(BaseParserModel):
     overrides: Optional[Overrides] = None
     depends_on: Optional[DependsOn12] = Field(None, title="DependsOn")
     config: Optional[Config53] = Field(None, title="UnitTestConfig")
-    checksum: Optional[str] = None
+    checksum: Optional[Union[str, Checksum]] = None
     schema_: Optional[str] = Field(None, alias="schema")
     created_at: Optional[float] = None
     versions: Optional[Versions] = None
@@ -4381,7 +4381,7 @@ class UnitTests(BaseParserModel):
     overrides: Optional[Overrides] = None
     depends_on: Optional[DependsOn12] = Field(None, title="DependsOn")
     config: Optional[Config60] = Field(None, title="UnitTestConfig")
-    checksum: Optional[str] = None
+    checksum: Optional[Union[str, Checksum]] = None
     schema_: Optional[str] = Field(None, alias="schema")
     created_at: Optional[float] = None
     versions: Optional[Versions] = None