fix(anthropic): prevent crash with cache_control and empty message content

[viktortnk]

Description: Fixes an IndexError: list index out of range in ChatAnthropic that occurs when using cache_control (e.g., for prompt caching) if the final message in the input sequence has an empty content list (e.g., AIMessage(content=[])).

Previously, _get_request_payload blindly accessed formatted_messages[-1]["content"][-1]. This PR updates the logic to iterate backwards through the formatted messages and attach cache_control to the first message with non-empty content, safely skipping empty blocks.

[codspeed-hq]

CodSpeed Performance Report

Merging #34025 will not alter performance

_{Comparing viktortnk:fix/anthropic-cache-control-index-error (c44b82c) with master (328ba36)}

Summary

✅ 3 untouched
⏩ 31 skipped¹

Footnotes

1. 31 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩