fix: Toolset.add drops tools from lazy child toolsets

[shivanij1203]

Summary

Toolset.add() warmed the object being added only when the parent was already warmed up. For a lazily-loaded child Toolset (one that materializes its tools in warm_up()), adding it to a not-yet-warmed parent meant it was flattened with list(tool) while still empty, so its tools were silently dropped and a later parent.warm_up() could not recover them. This contradicts the method's own comment about warming the source before flattening.

Fixes #12009

Changes

• haystack/tools/toolset.py: in Toolset.add(), warm a child Toolset before flattening it, regardless of the parent's warm-up state. Adding a plain Tool to a cold parent is unchanged (still warmed only when the parent is warmed up).
• test/tools/test_toolset.py: regression tests for adding a lazy child Toolset to a cold parent (tools retained) and for duplicate-name validation against the materialized tools.
• Release note under releasenotes/notes/.

Testing

• test/tools/test_toolset.py — 33 passed (31 existing + 2 new).
• Both new tests fail against the pre-fix code, so they guard the actual regression.
• ruff format --check and ruff check pass on the changed files.

Reproduction from the issue, before the fix:

lazy = LazyToolset()      # builds its tool in warm_up()
parent = Toolset()        # not warmed
parent.add(lazy)
# {'lazy_warmups': 0, 'parent_tools': []}  -> tools dropped

After the fix the child is warmed before flattening, so parent contains the lazy tool.

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[sjrl]

Closing as duplicate of #12012