Remove `enableTruncation` and `streamGenAiSpans` flag

[nicohrubec]

We'll remove these and the new recommendation for agent monitoring will be to enable span streaming if customers run into issues with transaction limits (if it's not enabled by default anyways). This tickets includes quite a bit of work so might be easier to still split this up into smaller issues/PRs.

Remove the enableTruncation flag:

• Remove the flag itself.
• Remove any text input truncation logic from all our integrations.
• TBD: Long term we want to send images as span attachments so if this is already ready we can get rid of the image truncation as well, but likely we'll still need image/binary data removal at this point so this will have to be refactored out and still applied.
• Remove any tests for text truncation.

Remove the streamGenAiSpans flag:

• Remove the flag itself.
• Remove any associated logic (removing gen_ai spans from transaction envelopes, transformation to v2 spans and wrapping up in a v2 envelope).
• Remove any tests that we have for this use case.

[linear-code]

JS-2593

[nicohrubec]

I have a stack of PRs prepared for this but we need to wait for internal feedback before we pull the trigger: #22548