fix: detect duplicate plugin entries in dependencies

[hashwnath]

Summary

Fixes #19

Problem

Currently, Concord configuration parsing silently merges or ignores duplicate entries in dependencies. If a user accidentally lists the same dependency twice (potentially with different versions), it can lead to confusion about which one is actually used.

Solution

Implemented a fail-fast validation in ProjectLoaderV2. The loader now checks for duplicate strings in dependencies and extraDependencies lists before merging them. If a duplicate is found, it throws an IllegalArgumentException with the message "Duplicate dependency found: ...".

Changes

• Modified ProjectLoaderV2.java to check for duplicates in parsed lists.
• Added testDuplicateDependencies to ProjectLoaderV2Test.java.
• Added test resource duplicate_deps/concord.yml.

Testing

• Ran mvn test -pl runtime/v2/model -Dtest=ProjectLoaderV2Test to verify the fix and ensure no regressions.
• Confirmed that valid configurations (like multi-file merges) still work as expected via existing tests.

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[hashwnath]

recheck

[benbroadaway]

This change has no practical benefit to workflow execution and will introduce breaking changes to existing flows. As discussed in #19 (comment), between policy overrides, imported flows, and transitive dependencies of workflow dependencies, duplicates are expected to be encountered.

If anything, a warning in the CLI linter may be appropriate for a heads up to flow designers they've introduced a duplicate in configuration.dependencies.