Fix Databricks operators with templated json payloads

[Vamsi-klu]

Fix Databricks operators so templated json payloads are rendered before the final Databricks request payload is built.

This updates DatabricksCreateJobsOperator, DatabricksSubmitRunOperator, and DatabricksRunNowOperator to:

• assign templated constructor fields directly in __init__
• build and validate the final payload in execute()
• accept rendered mappings, JSON strings, Python-literal dict strings, and XComArg values
• merge rendered named parameters after json, so named arguments keep precedence
• preserve existing validation for dbt/git source, pipeline id/name, job id/name, deferrable execution, and repair-run paths

This also adds regression coverage for templated named parameters and XComArg/classic Jinja payloads.

closes: #35433
related: #40788

Validation:

• uv run --project providers/databricks pytest providers/databricks/tests/unit/databricks/operators/test_databricks.py -xvs
• prek run --from-ref upstream/main --stage pre-commit
• prek run --from-ref upstream/main --stage manual
• breeze ci selective-check --commit-ref HEAD
• breeze testing providers-tests --test-type "Providers[databricks]"

Note: breeze selective-checks --commit-ref HEAD is documented in AGENTS.md/selective-checks docs, but the current Breeze command is breeze ci selective-check --commit-ref HEAD.

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Drafted-by: Codex (GPT-5) (no human review before posting)