Require configure before lazy remote variables

[alexmojaki]

Summary

• Require logfire.configure() before LOGFIRE_API_KEY can lazily enable the remote variable provider
• Keep lazy remote provider initialization after configuration when variables= is omitted
• Update managed-variable docs to describe the configure-first behavior

Tests

• uv run pytest tests/test_variables.py::TestLazyVariableProviderInit -q
• uv run ruff check logfire/_internal/config.py tests/test_variables.py
• Pre-commit via gcap: Ruff, Ruff Format, pyright

[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 3 files

Confidence score: 5/5

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

_{Re-trigger cubic}