CONV-221: Implement image to image cost

[menvil]

Summary

• Implements ConfigDrivenConversionCostEstimator::estimate() for image-to-image conversions (1 credit)
• Also prepares image-to-pdf path (2 credits) for CONV-223
• Adds UnsupportedConversionCostException for unknown conversion pairs

Test plan

• ConversionCostEstimatorTest passes (2 tests: PNG→JPG, JPG→WEBP = 1 credit)

🤖 Generated with Claude Code

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Summary by cubic

Implements cost estimation for image-to-image conversions in ConfigDrivenConversionCostEstimator (1 credit). Also sets up image→PDF rule (2 credits) and adds a clear exception for unsupported pairs, satisfying CONV-221.

• New Features
  • Image→image conversions now return a CreditCost with a 1-credit base; image→PDF path is wired via conversion_costs.rules.
  • Throws UnsupportedConversionCostException when a source/target pair isn't supported.
  • Uses conversion_costs.groups.image and .pdf to classify formats and includes a concise cost breakdown (rule, formats, converter key, file size).

^{Written for commit 9ea385e. Summary will update on new commits.}

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/CONV-221-implement-image-to-image-cost

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