Fix prompt stacking in bedrock converse

[GinkREAL]

Description

Bedrock converse edits self.system_prompt every time any chat function is called. This causes the system_prompt to keep bloating on its own due to the following chain of events:

• _extend_messages in llm.py adds the class level system prompt to the messages array
• messages_to_converse_messages combines the class level and chat level system prompt
• chat functions assign this combined system prompt back to the class level system prompt
• rinse and repeat, the class level system prompt endlessly grows

Fixes #17265

New Package?

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods

[logan-markewich]

Thanks!