Add usage report adapter layer

[asizikov]

Summary

Usage report parsing needs a format-aware entry point so current Transition Period Billing Preview reports keep working while newer native AI Credits reports can be identified separately. This PR adds that adapter layer without changing downstream normalization or aggregation behavior.

Commit	Change
refactor: add usage report adapter layer	Adds report format metadata, adapter selection, and first-row validation routing while preserving the existing Transition Period Billing Preview parser path and native AI Credits unsupported error.

Changes

• Adds transition-period-billing-preview and native-ai-credits report format metadata and adapters.
• Detects the report format from the parsed header plus the first non-empty data row.
• Routes pipeline preflight validation through the selected adapter while keeping immediate shared header validation for malformed or pre-AIC reports.
• Preserves native AI Credits behavior by detecting those reports and throwing UnsupportedNativeAiCreditsReportError for this slice.
• Adds focused adapter selection tests and pipeline header-only validation coverage.

Testing

• npm test
• npm run lint
• npm run build

Checklist

• Specs lint passes (npm run lint)
• Tests updated if applicable
• Documentation updated if applicable

[Copilot]

Pull request overview

This PR introduces a small adapter layer to make usage report preflight validation format-aware, so the existing Transition Period Billing Preview CSVs continue to work while “native AI Credits” CSVs are detected and rejected with the existing unsupported error—without changing downstream parsing/normalization/aggregation.

Changes:

• Adds reportAdapters with report-format metadata, detection (header + first data row), and adapter-selected first-row validation.
• Updates runPipeline preflight validation to use the adapter entry points while keeping header-only validation for malformed / pre-AIC reports.
• Adds targeted tests for adapter selection and for header-only report validation behavior in the pipeline.

Show a summary per file

File	Description
src/pipeline/runPipeline.ts	Routes preflight header + first-row validation through the new adapter entry points.
src/pipeline/runPipeline.test.ts	Adds coverage for valid/malformed/pre-AIC header-only reports and preserves native-report rejection behavior.
src/pipeline/reportAdapters.ts	New adapter layer: report format metadata, detection, adapter selection, and validation routing.
src/pipeline/reportAdapters.test.ts	New tests validating format detection/selection and early header validation behavior.
src/pipeline/parser.ts	Extracts native-report signature detection into hasNativeAiCreditsReportSignature and reuses it in validation.
src/pipeline/parser.test.ts	Renames variables for clarity (“legacyHeader” → “preAicHeader”) without behavior changes.

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• Files reviewed: 6/6 changed files
• Comments generated: 0