feat: expose raw batch failure details

[xxvw]

Summary

• add D1RawBatchException with the first failed item index and immutable raw results
• preserve item-level errors and messages while keeping existing HTTP and top-level mappings
• retain the attempted operation when successful-status responses contain invalid JSON
• document synchronous and asynchronous raw batch failure handling

Related Issue

Closes #107

Type of Change

• Feature
• Bug fix
• Test
• Documentation
• Refactor
• Build / CI
• Release

Checklist

• I created a branch before making changes.
• I did not commit directly to main.
• I used this pull request template.
• I linked the relevant issue where possible.
• I added or updated tests.
• I updated documentation if needed.
• Unreleased notes are consolidated in feat: improve environment-based client setup #113 to avoid cross-PR conflicts.
• I ran mvn clean verify.
• I did not include names of automated development tools in project artifacts.
• I did not expose API tokens, secrets, private keys, account IDs, database IDs, customer data, or sensitive data.
• I did not add new runtime dependencies without an issue and maintainer approval.

Testing

mvn -B clean verify
mvn -B -q javadoc:javadoc

109 tests passed with no failures, and Javadocs generated successfully.

Notes

D1RawBatchException is used only when an item in a successful raw batch response fails. HTTP failures and top-level API failures retain their existing exception mapping. Merge #113 after this pull request to add the consolidated changelog entries.