Fix Pydantic content lowering for text messages and tool results

[Kamilbenkirane]

Problem

Message.content accepts Pydantic models, and ToolResult inherits that contract through Message. Some request builders currently lower Pydantic content incorrectly when preparing provider payloads.

The important boundary is text message/tool-result lowering, not a generic celeste.tools serialization helper:

• Chat Completions and OpenResponses currently do message lowering from lower protocol tools.py modules that import Message / ToolResult.
• That lowering belongs in the text modality protocol layer.
• Anthropic, Google, and Cohere already lower messages in text provider clients and should be patched in place.

Use direct Pydantic APIs at the exact boundary:

• BaseModel.model_dump_json() for provider fields that require JSON text.
• BaseModel.model_dump(mode="json") for provider fields that accept JSON objects.
• model_dump(mode="json", serialize_as_any=True, exclude_none=True) when dumping a Message that may contain nested Pydantic content.

Do not add a generic Celeste serialization helper. Serialization remains owned by Pydantic.

Definition of Done

• Chat Completions serializes ToolResult(content=ToolOutput(...)) as JSON text.
• OpenResponses serializes ToolResult(content=ToolOutput(...)) as JSON text.
• Anthropic serializes ToolResult(content=ToolOutput(...)) as JSON text.
• Google emits JSON-compatible object content for Pydantic tool results.
• Cohere preserves nested Pydantic Message.content instead of dumping it as {}.
• Plain string tool results keep existing behavior.
• Focused tests cover the contract.