Spring-ai GH-1753: Add truncation support for Cohere embeddings

Fixes: #1753

https://github.com/spring-projects/spring-ai/issues/1753

• Add character-based truncation (max 2048 chars) for Cohere embedding requests
• Support both START and END truncation strategies
• Add unit tests verifying truncation behavior for both strategies

Truncation is applied before sending requests to Bedrock API to avoid ValidationException when text exceeds maximum length. The END strategy (default) keeps the first 2048 characters while START keeps the last 2048 characters.

Comment From: markpollack

merged in 1abfd9ab96a4f8499fe41dccc2f95d5766da985b