fix(skills): preserve sibling parts when loading binary resources

[quifox]

Description of Change

Problem: When load_skill_resource expands a binary function response in the final request content, its request processor drops every other part from that content. For example, a binary resource response and a sibling tool response merged into one content become only the binary resource response and its inline data.

Solution: Preserve parts that are not binary responses from this tool. Keep the existing binary expansion in place and retain the original ordering of text, sibling responses, and existing inline data. Add regression coverage for mixed parts, repeated processing, and content with no binary resource response.

Testing Plan

Unit Tests:

• I have added unit tests for my change.
• New and existing core unit tests pass with my changes.

Java 17, with Google Java formatting enabled:

mvn -B -ntp -pl core -Dfmt.skip=false -Dtest=LoadSkillResourceMixedPartsRegressionTest,LoadSkillResourceToolTest -Dsurefire.failIfNoSpecifiedTests=false test
mvn -B -ntp -pl core -Dfmt.skip=false test

The focused suite passed 15 tests in each of the default-test and basic executions. Full core completed with 2,017 tests, 0 failures, 0 errors, and 24 skipped in each of those two executions. The additional configured Surefire executions also completed successfully. The two main executions run the same suite; their counts are not added together as distinct tests.

Manual End-to-End (E2E) Tests: Not run against a live model provider. The unit tests exercise the request processor directly with a mixed content payload and verify preserved order, function-response IDs, binary bytes, MIME type, and stable repeated processing.

Checklist

• I have read CONTRIBUTING.md.
• My pull request contains a single commit.
• I have performed a self-review of my code.
• I have added tests that prove the fix is effective.
• New and existing core unit tests pass with my changes.
• I have manually tested my changes end-to-end.

No new dependencies or public APIs are introduced.

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