fix: Serverless Python model environment version configuration (#1277)

[benc-db]

Resolves #1277

Description

• Replace deprecated 'client' field with 'environment_version' in environment spec for serverless Python models
• Add JobEnvironment SDK object conversion in api_client.py to fix 'dict object has no attribute as_dict' error
• Enable TestServerlessClusterWithEnvironment functional test (was previously skipped as 'not available in Databricks yet')
• Add unit tests for environment_version usage and user-provided environments override behavior
• Users can now specify custom environment versions via python_job_config.environments

Checklist

• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [ ] I have updated the CHANGELOG.md and added information about my change to the "dbt-databricks next" section.

[github-actions[bot]]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
dbt/adapters/databricks
api_client.py					536
dbt/adapters/databricks/python_models
python_submissions.py
Project Total

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