fix: keep title keys inside OpenAPI singular example values

[LHMQ878]

Summary

• Follow-up to fix: don't strip title when it is a name, not a schema keyword #12219: treat OpenAPI 3.0 singular example as instance data in _DATA_SCHEMA_KEYWORDS, same as examples / default / const / enum.
• Keeps nested title keys carried via Pydantic json_schema_extra={example: ...} when building tool schemas.
• Adds a unit fixture that discriminates singular example and a public-API regression through create_tool_from_function.

Context

Flagged by @percymcn on #12219 after re-checking the post-merge 3.1.0.dev build.

Test plan

• test_remove_title_from_schema_keeps_instance_data passes with singular example
• test_from_function_with_openapi_example_containing_title_key passes
• Existing title-stripping controls (items / propertyNames) still remove schema-level title

[vercel]

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[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/tools
from_function.py
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