LangChain: Fix message timestamps, add default role assignment, and Bedrock support

[sgoel-nr]

Summary

This PR addresses several bugs related to LangChain integration in the New Relic Python Agent, specifically focusing on message timestamp handling and role assignments for chat completion events.

Changes

Message Timestamp Handling

• Fixed request message timestamp preservation for LangChain chat completion messages
• Ensured proper timestamp propagation to LlmChatCompletionMessage events
• Corrected request timestamp handling for LlmChatCompletionSummary table entries

Role Assignment

• Implemented default role assignment for input/output messages within LangChain
• Ensures consistent role metadata across chat completion events

Bedrock Integration

• Bedrock methods are properly integrated with the LangChain instrumentation

[CLAassistant]

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2 out of 3 committers have signed the CLA.

✅ bonczj
✅ sgoel-nr
❌ mergify[bot]
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.35849% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.28%. Comparing base (8d18c0a) to head (43cb0b4).

Files with missing lines	Patch %	Lines
newrelic/hooks/external_botocore.py	66.66%	9 Missing ⚠️
newrelic/hooks/mlmodel_langchain.py	71.42%	0 Missing and 2 partials ⚠️
newrelic/hooks/mlmodel_gemini.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1580      +/-   ##
==========================================
- Coverage   81.82%   81.28%   -0.54%     
==========================================
  Files         207      207              
  Lines       23953    23979      +26     
  Branches     3799     3804       +5     
==========================================
- Hits        19599    19491     -108     
- Misses       3086     3231     +145     
+ Partials     1268     1257      -11     

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