Expose ResolvedVariable reason

[alexmojaki]

Summary

• expose ResolutionReason and make ResolvedVariable.reason public
• include code_default as the final resolution reason when a variable falls back to its code default
• rename internal _reason usage to reason across variable providers and tests

Extracted from #1731 without the unrelated variable composition changes.

Tests

• uv run pytest tests/test_variables.py
• uv run ruff check logfire/variables/init.py logfire/variables/abstract.py logfire/variables/config.py logfire/variables/remote.py logfire/variables/variable.py tests/test_variables.py
• uv run pyright logfire/variables/abstract.py logfire/variables/config.py logfire/variables/remote.py logfire/variables/variable.py tests/test_variables.py

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View logs

[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.

[cubic-dev-ai]

No issues found across 6 files

Confidence score: 5/5

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

_{Re-trigger cubic}

[cubic-dev-ai]

1 issue found across 3 files (changes from recent commits).

<sub>Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic</sub>

[alexmojaki]

so was the missing label/version just a bug in main?

[alexmojaki]

(AI) Yes, that looks like a pre-existing bug in main: the validation-error path already fell back to the code default while preserving the exception, but it rebuilt ResolvedVariable without carrying over the selected provider label/version. This PR fixes that while doing the _reason -> reason rename.