fix: make nested response values immutable

[xxvw]

Summary

• recursively freeze JSON-shaped maps and lists exposed by public response models
• defensively copy nested values in result rows, metadata, response information, and mapping failures
• reject cyclic map/list inputs with a clear validation error
• update immutability requirements and regression coverage

Related Issue

Closes #108

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

111 tests passed with no failures.

Notes

The recursive copy is intentionally limited to JSON-shaped Map and List containers. Scalar values retain their existing representation. Merge #113 after this pull request to add the consolidated changelog entry.