ci: fix incremental build exclusion list failing on leaf modules

[Croway]

Summary

• Fix Could not find the selected project in the reactor: :camel-allcomponents error that breaks CI for PRs changing leaf modules (e.g. camel-jbang)
• The EXCLUSION_LIST appended to -pl with -amd references modules that may not be dependents of the changed modules, causing Maven to fail when they're not in the resolved reactor
• Resolve the -amd reactor and drop exclusions for modules not present in it
• Reuse the threshold check's Maven call to capture reactor artifact IDs, avoiding a second invocation

Root cause: introduced in #22247 (Apr 8) — the EXCLUSION_LIST was appended to -pl -amd without validating that excluded modules are reachable as dependents. Works for core modules (whose dependents include everything) but fails for leaf modules with small reactors.

Reproducer: ./mvnw -B -q help:evaluate -Dexpression=project.artifactId -pl "dsl/camel-jbang/camel-jbang-core,!:camel-allcomponents" -amd

Test plan

• Reproduced locally: the above command fails before the fix
• Verified camel-allcomponents is correctly identified as NOT in the -amd reactor for camel-jbang-core
• Verified camel-allcomponents IS in the -amd reactor for core/camel-api (exclusion kept)
• CI passes on a PR that changes only leaf modules (e.g. camel-jbang)

[github-actions]

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