fix: preserve lazy toolsets added before warm-up

[Success6666]

Related Issues

• fixes Toolset.add() silently drops tools from lazy Toolsets before parent warm-up #12009

Proposed Changes:

Toolset.add() only warmed the object being added when the parent had already been warmed. A lazy child Toolset was therefore flattened while still empty when added to a cold parent, and its tools could not be recovered later.

• Warm child Toolset instances before flattening them.
• Keep the existing cold-parent behavior for plain Tool instances.
• Add regression coverage for lazy loading, idempotent warm-up, and duplicate-name validation.
• Add a release note for the behavior fix.

How did you test it?

• hatch run test:unit test/tools/test_toolset.py -q (33 passed)
• hatch run test:types (Success: no issues found in 371 source files)
• hatch run pre-commit run --files haystack/tools/toolset.py test/tools/test_toolset.py releasenotes/notes/fix-toolset-add-lazy-c73a533f031c72d2.yaml

Notes for the reviewer

The change intentionally warms only child Toolset instances before flattening. Adding a plain Tool to a cold parent remains deferred until the parent is warmed.

This PR was fully generated with an AI assistant. The changes were reviewed against the affected code path and the relevant tests were run locally.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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