Refactor inference data_writer configs to the Builder pattern

[mcgibbon]

Corrects the Builder-pattern-pass violations in fme/ace/inference/data_writer/ flagged by the 2026-06-24 builder-pattern audit, so the config schema (the dacite/YAML surface) is decoupled from the runtime writer implementations. The change is behavior-preserving. Follows the file-ordering / leaf-*Params template set by the corrector refactor (#1313).

The audit flagged three issues here:

• Rule 2 — DataWriterConfig.build/build_paired passed raw time_coarsen/files sub-configs into DataWriter/PairedDataWriter, which then built the sub-writers themselves.
• Rule 1 (non-leaf impl read) — FileWriter received the whole non-leaf FileWriterConfig and read ~7 of its fields.
• Rule 1 (parent read) — InferenceConfig/InferenceEvaluatorConfig.__post_init__ reached into data_writer.time_coarsen and iterated data_writer.files / file_config.time_coarsen.

Changes:

• fme.ace.inference.data_writer.main.DataWriterConfig: build/build_paired construct the full list of sub-writers (raw/monthly/file writers, plus the time-coarsen wrapper) and pass them in; DataWriter/PairedDataWriter.__init__ now take a pre-built writers list instead of raw sub-configs + enable flags. Added validate_time_coarsen() as the sanctioned accessor for parent configs.
• fme.ace.inference.data_writer.file_writer: added FileWriterParams, a plain leaf params dataclass; FileWriter takes it (stores self._params, never self._config) instead of the non-leaf FileWriterConfig. Added FileWriterConfig.validate_time_coarsen() and _build_writer_params().
• fme.ace.inference.inference.InferenceConfig / fme.ace.inference.evaluator.InferenceEvaluatorConfig: __post_init__ now calls data_writer.validate_time_coarsen(...) instead of reading the raw fields.
• Tests: build writers through DataWriterConfig.build/build_paired and FileWriterConfig._build_writer_params rather than the old impl constructor signatures.

Notes:

• FileWriterParams.time_selection holds the selector sub-configs (Slice | MonthSelector | TimeSlice), so the struct is a "runtime params" leaf in spirit (it forwards an opaque selector straight to _select_time, which dispatches via each selector's own methods — no raw cross-config field reads) rather than a strict field-type leaf. The alternative (re-deriving the selection inside the builder) would duplicate _select_time's dispatch.

• The non-paired DataWriter's HEALPix (face) early-return is preserved in DataWriterConfig.build (it produces an empty writer list). path/metadata are now always stored, which only affects the never-used-in-production HEALPix DataWriter.write path (previously would AttributeError; both inference entrypoints use build_paired, which never had a face check).

• The coupled CoupledDataWriterConfig parent reads of data_writer.{ocean,atmosphere}.time_coarsen are a separate audit section (they wrap validation in per-component try/except messaging) and are left as a follow-up; this refactor keeps time_coarsen a public field so nothing there breaks.

• Tests added (existing data_writer tests migrated to the public builder API; behavior coverage preserved)

Part of the builder-pattern-pass effort (task 2026-06-24-refactor-data-writer-configs-builder-pattern, goal 2026-06-24-ace-obeys-builder-pattern-pass).