Address variable structure review findings

[alexmojaki]

Stacked on #1947.

Addresses the review comments about:

• preserving provider exceptions when rendered code defaults are used
• avoiding serialized-default work when no render hook is present
• treating Pydantic validator TypeError as a deserialization failure while preserving label/version metadata

Tests: uv run pytest tests/test_variables.py -q

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.

[dmontagu]

Superseded by #1731 — cherry-picked onto the variable-composition branch as 8b04bfd (resolving the conflict with #1731's later revert of the early-return that broke composition for plain Variable defaults containing @{...}@ references). Closing in favor of #1731.