fix: null dbt_project.yml keys cause state:modified to over-select siblings

[wilu222]

Resolves #16290

Problem

If a key in the dbt_project.yml model tree has a null value (e.g. beta: with no children or configs), any change to that key caused state:modified to select every model beneath the parent block, including unrelated siblings.

Root cause: _project_configs treated any non-dict value as a legacy bare config. YAML null parses as None, which is not a dict, so the placeholder key was copied into every sibling's unrendered_config and then compared by same_contents.

Solution

Skip None values in the legacy bare-config branch so null hierarchy placeholders are not treated as config, while still supporting bare scalars (materialized: view) and +-prefixed explicit nulls (+enabled: null).

Adds unit regression coverage in tests/unit/context/test_context_config.py.

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.

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[Copilot]

🔵 Needs a closer look

Wrap the test signature exceeding the configured 99-character line length.

Pull request overview

Fixes false state:modified selections caused by null-valued keys in dbt_project.yml.

Changes:

• Ignores unprefixed null hierarchy placeholders.
• Preserves scalar and explicit +-prefixed null configs.
• Adds regression tests and a changelog entry.

File summaries

File	Summary
tests/unit/context/test_context_config.py	Adds regression coverage.
core/dbt/context/context_config.py	Skips null hierarchy placeholders during config extraction.
.changes/unreleased/Fixes-20260911-214018.yaml	Documents the fix.

Review details

Suppressed comments (1)

tests/unit/context/test_context_config.py:8

• This new signature is 108 characters, exceeding dbt-core's configured Black line length of 99, so the code-quality formatter will reject or rewrite this file. Please wrap the signature across lines.

def _project_config_levels(model_configs: Dict[str, Any], fqn: List[str]) -> List[Dict[str, Any]]:

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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[wilu222]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[wilu222]

This line is 98 characters; black’s limit in this repo is 99. Black already passed locally and left it as one line. Copilot's 108 count looks off. No wrap needed unless a maintainer prefers it.