refactor: use Prompt Registry for code generation

Relates to issue #1725

Changes

This MR refactors code generation to use the Prompt Registry when there is no prompt id, which is a legacy case (pre-Oct. 2024). This legacy path was using Claude Sonnet v3.5 (anthropic) or Code Bison (vertex-ai) before, so it's unlikely anyone was actually using this. This is simply to support removing legacy code

How to Verify

# Default case (vertex)
curl -X "POST" "http://localhost:5052/v3/code/completions" \
     -H 'X-Gitlab-Global-User-Id: test_id' \
     -H 'X-Gitlab-Model-Prompt-Cache-Enabled: true' \
     -H 'X-gitlab-Realm: SM' \
     -H 'Content-Type: application/json; charset=utf-8' \
     -H 'Cookie: BetterErrors-2.10.1-CSRF-Token=759b9551-abff-4559-a2d7-336c212b0d4e' \
     -d $'{
  "prompt_components": [
    {
      "type": "code_editor_generation",
      "payload": {
        "language_identifier": "go",
        "file_name": "test",
        "content_below_cursor": "\\n}",
        "content_above_cursor": "func hello_world(){\\n\\t"
      }
    }
  ]
}'

# Anthropic specified case
## local AIGW completion v3
curl -X "POST" "http://localhost:5052/v3/code/completions" \
     -H 'X-Gitlab-Global-User-Id: test_id' \
     -H 'X-Gitlab-Model-Prompt-Cache-Enabled: true' \
     -H 'X-gitlab-Realm: SM' \
     -H 'Content-Type: application/json; charset=utf-8' \
     -H 'Cookie: BetterErrors-2.10.1-CSRF-Token=759b9551-abff-4559-a2d7-336c212b0d4e' \
     -d $'{
  "prompt_components": [
    {
      "type": "code_editor_generation",
      "payload": {
        "language_identifier": "go",
        "model_provider": "anthropic",
        "file_name": "test",
        "content_below_cursor": "\\n}",
        "content_above_cursor": "func hello_world(){\\n\\t"
      }
    }
  ]
}'

# Vertex specified case
## local AIGW completion v3
curl -X "POST" "http://localhost:5052/v3/code/completions" \
     -H 'X-Gitlab-Global-User-Id: test_id' \
     -H 'X-Gitlab-Model-Prompt-Cache-Enabled: true' \
     -H 'X-gitlab-Realm: SM' \
     -H 'Content-Type: application/json; charset=utf-8' \
     -H 'Cookie: BetterErrors-2.10.1-CSRF-Token=759b9551-abff-4559-a2d7-336c212b0d4e' \
     -d $'{
  "prompt_components": [
    {
      "type": "code_editor_generation",
      "payload": {
        "language_identifier": "go",
        "model_provider": "vertex-ai",
        "file_name": "test",
        "content_below_cursor": "\\n}",
        "content_above_cursor": "func hello_world(){\\n\\t"
      }
    }
  ]
}'