Aside from introducing unit tests for the related_models module, we also changed
the way missing back-references are represented.

Before each missing back-reference name is generated using the following
template: `__dummy_${model}_${dummyNameId++}_${suffix}__`

Now, they're computed using: `<-${model}.${fieldName}`.

E.g.: given the following models:

```js
createRelatedModels({
    "product.product": {
        tag_ids: {
            type: "many2many",
            relation: "product.tag",
            relation_table: "product_tag_product_product_rel",
        },
    },
    "product.tag": {},
})
```

Each `product.tag` object will automatically be assigned a field called
`<-product.product.tag_ids` which points to the `product.product` objects
related to the `tag_ids`. Check the following test module for better
illustration: `models without backlinks > many2many relations`

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## Purpose
It's often the case that it is necessary to check for the existence of records of a specific model that matches a domain predicate, usually done during python validation for enforcing constraints. What is done currently is either a `search` with a `limit=1`, which is not optimal, as the `_order` is applied, in better case `search_count` is used instead.
The issue is mostly a question of expressiveness, as `search_count(.. ., limit=1)` is not really obvious for what we are checking: existence.

## Feature
Create a new model API endpoint `search_exists`, that takes a `domain` and check if there are records in the model that satisfy the domain.

## Implementation
Check the existence with `SELECT EXISTS(...)` as it's the most trivial plan possible for what we are trying to do (InitPlan -> Results). Note that the presence of a limit, if it's not 0, will usually be optimized out by [postgres][1], so the presence of it on the query object is irrelevant.

[1]:https://github.com/postgres/postgres/blob/0de5274f59c9d84d803d7f3dc75577d8aa6e56b8/src/backend/optimizer/plan/subselect.c#L1523-L1611

## Reference
task-3844634

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