fix(ai): correct size calculation, rename internal property for message truncation & add test

[shellmayr]

• Fixes an issue with size calculation - the meta tag should show the original amount of messages and not the original size of the output string
• Renames the internal property to be more semantically correct - it contains the original length, not the number of truncated messages
• Add a test for the content of the meta tag
  Contributes to TET-1208

[linear]

TET-1208 Python SDK: truncate gen_ai messages