[BUG] POST messages API overwrites participant's configured LLM model with project default

[umishra1504]

Description

When creating a message in a conversation using an agent or pipeline as a participant via the POST messages API, the system ignores the LLM model configured in the participant (agent/pipeline) and instead uses the project-level default model. This causes unexpected behavior where conversations run with a different model than intended.

Steps to Reproduce

1. Create a conversation:

curl --location 'https://stage.elitea.ai/api/v2/elitea_core/conversations/prompt_lib/{{project_id}}' \
--header 'Authorization: Bearer <token>' \
--data '{
  "name": "Pipeline Participants",
  "is_private": true,
  "participants": []
}'

2. Add a participant (pipeline or agent with specific model configuration):

curl --location 'https://stage.elitea.ai/api/v2/elitea_core/participants/prompt_lib/{{project_id}}/{{conversation_id}}' \
--header 'Authorization: Bearer <token>' \
--data '{
    "entity_name": "application",
    "entity_meta": {
        "id": <agent_id>,
        "name": "<agent name>",
        "project_id": <project_id>
    },
    "entity_settings": {
        "agent_type": "pipeline",
        "version_id": <agent_version_id>
    },
    "meta": {}
}'

3. Send a message to the conversation:

curl --location 'https://stage.elitea.ai/api/v2/elitea_core/messages/prompt_lib/{{project_id}}/{{conversation_uuid}}' \
--header 'Authorization: Bearer <token>' \
--data '{
    "participant_id": "{{paritcipants_id}}",
    "await_task_timeout": -1,
    "user_input": "go",
    "return_task_id": "true"
}'

Actual Result

• The message execution uses the project-level default model (e.g., "GPT-5 mini" as shown in Settings > AI Configuration)
• The participant's configured model (set in the pipeline/agent configuration) is ignored/overwritten

Expected Result

• The message execution should use the LLM model configured in the participant (pipeline ID: 239, version ID: 330)
• Project-level default model should only be used if no model is explicitly configured in the participant

Test Data

Pipeline link: https://stage.elitea.ai/app/32/pipelines/all/239?viewMode=owner&name=AgentNode

[umishra1504]

cc. @bermudas