Conditionally enable OpenRewrite FQCN shortening in CI

[gnodet]

Follow-up to #23041 which disabled OpenRewrite entirely because it quadrupled CI build times.

This re-enables OpenRewrite conditionally: regen.sh now greps the diff for fully qualified class name patterns and only adds -Prewrite when FQCNs are detected. Most builds (dependency bumps, doc changes, code without FQCNs) skip it entirely.

How it works

Before the build, the script:

1. Ensures the parent commit is available (git fetch --deepen=1 for shallow clones)
2. Diffs HEAD~1 HEAD on .java files (excluding generated, imports, packages, comments)
3. If any FQCN pattern is found (e.g. java.util.List, org.apache.camel.Exchange), appends -Prewrite to MAVEN_EXTRA_ARGS
4. The existing mvn install picks it up — no separate build pass needed

For PR builds, actions/checkout creates a merge commit where HEAD~1 is the base branch tip, so the diff covers all PR changes. For main builds, HEAD~1 is the previous (squash-merged) commit.

Test plan

• Tested regex locally: correctly detects FQCNs in commits that have them
• Correctly skips commits without FQCNs (dep bumps, FQCN removals)
• Import/package/comment lines are filtered out (no false triggers)
• Handles shallow clones (--deepen=1 to get parent commit)
• All fetch failures are gracefully handled (falls back to skipping OpenRewrite)
• Verify CI passes with this change

[github-actions]

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[github-actions]

🧪 CI tested the following changed modules:

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[gnodet]

Claude Code on behalf of Guillaume Nodet

Benchmark results

Tested with two companion PRs to measure the impact:

PR	OpenRewrite	Build time (JDK 21)
#23068 (no FQCNs)	skipped	~22 min
#23069 (with FQCNs)	triggered	~30 min

~8 minutes saved (~36% faster) on builds that don't introduce FQCNs, which is the common case (dependency bumps, doc changes, clean code).

When FQCNs are detected, OpenRewrite runs as before — no regression in correctness.