test: validate real builder configs in ci

[Scr4tch587]

What changed

New `tests/core/runtime/test_real_scripts.py` that validates the real `builders/scripts/` directory (not fixtures):

• runs `load_all_configs()` over the actual scripts dir — the exact validation the server runs at startup (structure, deps exist, cycle check, granularity, start-date constraints)
• per-dataset parametrized check that each `builder.py` exists, parses, and defines a top-level `build(dependencies, timestamp)` with the right arity (AST-only, so builder dependencies are never imported and no `.env` is needed)

Why

Until now, nothing in CI validated the real scripts directory — an invalid `config.toml` merged to main would only surface as a server startup failure at the next deploy. With the upcoming dataset-creation flow opening PRs that add dataset directories, CI needs to be the reviewer's guarantee.

Benefit

Green CI on a dataset PR now means: this dataset cannot break server boot. Reviewers can focus on builder logic instead of config mechanics.

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