Add codestral prompt for code completions

What does this MR do and why?

This MR adds support for using a codestral model for Code Completions via Code Suggestions.

It does so by:

1. Create a new abstract class AiGatewayCodeCompletionMessage requiring only the prompt method to be implemented by subclasses.
2. Create the respective CodestralMessage subclass that implements the FIM prompt required by Codestral.
3. Refactor CodeGemmaMessage to also subclass AiGatewayCodeCompletionMessage, removing some duplicated code.

Part of #467433 (closed)

Evaluation results

I ran the code-suggestions eval pipeline against a local gdk instance that uses codestral for code completions.

config

{
  "beam_config": {
    "pipeline_options": {
      "runner": "DirectRunner",
      "project": "dev-ai-research-0e2f8974",
      "region": "us-central1",
      "temp_location": "gs://prompt-library/tmp/",
      "save_main_session": false
    }
  },
  "input_bq_table": "dev-ai-research-0e2f8974.code_suggestion.input_testcases_v1",
  "output_bq_table": "dev-ai-research-0e2f8974.code_suggestion_experiments.bc_codestral_completions_py_only_temp0",
  "throttle_sec": 1,
  "batch_size": 10,
  "min_length": 0,
  "eval_setups": [
    {
      "llm_config": {
        "name": "gitlab-code-completion",
        "parameters": {
          "base_url": "http://gdk.test:3000",
          "intent": "completion"
        }
      },
      "languages": [
        "python"
      ],
      "post_transformations": [],
      "include_suffix": true
    }
  ]
}

model	avg_similarity_score	sample_size	table
codestral:latest	0.77	510	dev-ai-research-0e2f8974.code_suggestion_experiments.bc_codestral_completions_py_only_temp0
prod (code-gecko)	0.79	510	dev-ai-research-0e2f8974.code_suggestion_experiments.bc_becko_completions_py_only

Example query:

SELECT model, round(avg(similarity_score), 2) as avg_score, count(*) FROM `dev-ai-research-0e2f8974.code_suggestion_experiments.bc_codestral_completions_py_only_temp0` WHERE NOT STARTS_WITH(model_completion, '{') GROUP BY model

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Before	After

How to set up and validate locally

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