Bedrock - use `reasoning` field in `StreamingChunk`

[sjrl]

Refer to parent issue for more details

[Br1an67]

I'd like to work on this. Populate StreamingChunk.reasoning with ReasoningContent instead of storing reasoning content deltas in meta["reasoning_contents"], aligning with the pattern from the Ollama integration (#2850) and Google GenAI (#2900).

PR: deepset-ai/haystack-core-integrations#2901