Enable/refine reasoning support

In #9696, we added the ReasoningContent field to ChatMessage for storing model's reasoning.

For each ChatGenerator that explicitly supports reasoning-capable models, we should do the following:

• Verify whether init/run parameters allow reasoning settings. If not, add them.
• Refactor internal methods to ensure that reasoning is stored in the new field (not in meta).
• At runtime, handle reasoning content appropriately: some providers require removing reasoning from previous messages, while others require including it.
• Check how reasoning works with tool calls. Several providers have special requirements (e.g., using reasoning signatures).
• Check that when streaming is enabled, the reasoning content is successfully returned in the final assistant message. (For the moment, I have the impression that we cannot support streaming reasoning in a unified way).

Most child issues should wait until the 2.17.0 release (which will include the new field).

The list of child issues might not be comprehensive. If I forgot some relevant ChatGenerator, feel free to add it.

Start by reading #9696 and locating ChatMessage plus the ChatGenerator implementations that support reasoning-capable models. Inventory their init/run parameters, message handling, tool-call behavior, and streaming paths; done means reasoning uses the new field consistently and provider-specific requirements are covered.