fix(config): decouple ignore datasets fetching from config loading

[lewisjared]

Description

Resolves #549. This is a non-breaking re-cut of the salvageable core of #622, which had drifted too far behind main to rebase.

Decouple fetching from Config construction. Config.default() no longer performs network I/O as a side effect: the field factory for ignore_datasets_file is now a pure path computation, and the download moves to a new refresh_ignore_datasets_file() helper called lazily from ExecutionSolver.build_from_db at solve time. Read-only commands such as ref providers list and ref datasets list never touch the network. This also removes the __validation_ignore_datasets.yaml shim from Config.validate(), which only existed to dodge the side-effecting factory.

New Config.ignore_datasets_url field (env REF_IGNORE_DATASETS_URL) controls where the file is fetched from. Setting it to an empty string disables fetching entirely, which is what offline / air-gapped HPC deployments need. ignore_datasets_file is also now overridable via REF_IGNORE_DATASETS_FILE and coerced to a Path.

Fail-safe refresh. A failed download no longer creates an empty placeholder file (which previously suppressed the ignore list for six hours): an existing cached copy is reused unchanged, and a solve with no cached copy raises IgnoreDatasetsRefreshError rather than silently running without ignore-dataset protections.

Deterministic tests. Both test config fixtures now pin the in-tree default_ignore_datasets.yaml with fetching disabled, so tests exercise the shipped default file instead of whatever copy happens to be cached on the host, and a PR editing the file will correctly ripple into test results (the root cause of #590's cached-copy problem).

The docs gain a "Grey list" section describing the mechanism, the lazy fetch behaviour, and the offline workflow. The section notes that the ignore_datasets_* settings will be renamed to grey_list_* in a future release; the rename deliberately stays out of this PR to keep it non-breaking.

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

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📥 Commits

Reviewing files that changed from the base of the PR and between 67f1580 and e1942f4.

📒 Files selected for processing (9)

• changelog/792.fix.md
• docs/configuration.md
• packages/climate-ref-core/src/climate_ref_core/exceptions.py
• packages/climate-ref/src/climate_ref/config.py
• packages/climate-ref/src/climate_ref/conftest_plugin.py
• packages/climate-ref/src/climate_ref/solver.py
• packages/climate-ref/tests/unit/test_config.py
• packages/climate-ref/tests/unit/test_provider_registry.py
• packages/climate-ref/tests/unit/test_solver.py

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• Create PR with unit tests
• Commit unit tests in branch fix/ignore-datasets-decouple

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[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
packages/climate-ref/src/climate_ref/config.py	92.85%	1 Missing and 1 partial ⚠️

Flag	Coverage Δ
core	92.44% <93.75%> (-0.02%)	⬇️
providers	86.97% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...limate-ref-core/src/climate_ref_core/exceptions.py	97.72% <100.00%> (+0.10%)	⬆️
packages/climate-ref/src/climate_ref/solver.py	98.93% <100.00%> (+<0.01%)	⬆️
packages/climate-ref/src/climate_ref/config.py	96.52% <92.85%> (-0.85%)	⬇️

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