fix bug #2513(vertexai): improve input attribute handling for prompts and conte…

[kowshik24]

Fix Bug Report: Whole message history is stringified into a single 'user prompt' #2513

1. Bug Analysis
   Current Behavior
   Messages from chat history are combined into single string
   All roles and content merged into one user prompt
   Loses message separation and role information

2. Root Cause
   _set_input_attributes() treating Content list as plain text
   Not properly parsing individual Content objects
   Missing logic to handle chat message format

3. Impact
   Loss of conversation structure in telemetry
   Incorrect attribution of message roles
   Poor observability of chat interactions

4. Fix Required
   Update _set_input_attributes() to handle Content objects
   Parse role and parts from each message
   Store each message separately with correct role
   Maintain message order with index

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