Keep tool parameters named title in generated input_schema, closes #1679

[cpruijsen]

Fixes:

• Tool parameter named "title" is stripped from input_schema but kept in required #1679

A tool function with a parameter named title had that parameter deleted from the generated input_schema properties while required still listed it. Schema-strict models then omit the argument and the call fails with TypeError: ... missing 1 required positional argument: 'title'.

Tatsuhiko Miyagawa reported this on llm 0.26 and 0.35 and traced it to _remove_titles_recursively(), which is meant to strip Pydantic's "title": "When" annotations. Recursion into properties (and $defs) treated a property named title as that keyword and deleted the whole field. The same tool had been working on models that were willing to go off-schema; it broke on gpt-5.6-luna.

The helper now pops title only when the value is a string, which is what JSON Schema's title keyword is. A property or $defs entry named title has an object value, so it is kept. Pydantic annotations on sibling fields are still removed.

I chose the string-value check rather than skipping properties/$defs name-maps (the other fix suggested on the issue) or popping only when the dict also has type/anyOf. The string check is one condition, matches the spec, and covers nested models without a keyword list. Happy to switch to the name-map walker if preferred.