fix(renovate-deps): surface real renovate failure in error output

[zeitlinger]

Summary

• The renovate-deps linter previously truncated the renovate log to the first 20 lines on failure, which on a typical run are debug/info startup messages — the actual fatal error never made it into the bail message.
• Prefer JSON log entries with level >= 40 (warn/error/fatal), including err.message when present.
• Fall back to the last 20 lines when no high-level entries exist.

Motivation

Reported by Jay: flint run --fix against prometheus/client_java failed with renovate exited with status 1 followed by 20 lines of level":20 debug output, with no indication of what actually went wrong.

Test plan

• cargo test --bin flint extract_failure — both new unit tests pass
• cargo clippy --all-targets --no-deps
• Re-run renovate against a known-failing repo and confirm the surfaced snippet contains the fatal/error line

[Copilot]

Pull request overview

Improves renovate-deps linter failure reporting by extracting a more informative error snippet from Renovate’s JSON logs, so fatal/warn/error lines are surfaced instead of mostly startup debug output.

Changes:

• Replace the “first 20 lines of log” truncation with extract_failure_snippet() that prefers level >= 40 JSON entries and otherwise falls back to the last 20 lines.
• Add unit tests covering both the “prefer error lines” and “tail fallback” behaviors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/linters/renovate_deps/mod.rs	Adds extract_failure_snippet() and uses it in the Renovate failure bail message.
src/linters/renovate_deps/tests.rs	Adds unit tests validating the new snippet extraction behavior.

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