support sql_header config for data tests

[aahel]

Resolves #9775

Problem

The sql_header config is not supported for data tests (singular and generic). When users configure sql_header — either via the set_sql_header macro in singular tests or via YAML config: in generic tests — the value is silently dropped during parsing because:

1. TestConfig has no sql_header field, so any value set during parsing is discarded during serialization.
2. TestBuilder.CONFIG_ARGS does not include sql_header, so generic tests don't recognize it as a config key from YAML and it gets treated as a test argument instead.

This prevents users from prepending SQL statements before test execution — a capability that already works for models.

Solution

Add sql_header as a recognized config field for data tests:

• Add sql_header: Optional[str] = None to TestConfig dataclass and include it in the same_contents modifiers list for partial parsing support.
• Add "sql_header" to TestBuilder.CONFIG_ARGS so generic tests accept sql_header in their YAML config block.

Note: A companion PR in dbt-adapters is needed to actually prepend the sql_header in the test and unit test materialization macros (test.sql and unit.sql). Without that change, the config value is correctly parsed and stored but not applied during execution.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

[github-actions]

Additional Artifact Review Required

Changes to artifact directory files requires at least 2 approvals from core team members.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.37%. Comparing base (019ebdc) to head (c3fb292).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12545      +/-   ##
==========================================
- Coverage   91.45%   91.37%   -0.09%     
==========================================
  Files         203      203              
  Lines       25471    25551      +80     
==========================================
+ Hits        23294    23346      +52     
- Misses       2177     2205      +28     

Flag	Coverage Δ
integration	88.18% <100.00%> (-0.16%)	⬇️
unit	65.41% <100.00%> (-0.07%)	⬇️

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Components	Coverage Δ
Unit Tests	65.41% <100.00%> (-0.07%)	⬇️
Integration Tests	88.18% <100.00%> (-0.16%)	⬇️

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