## Description

This PR removes as many constraints as possible from Backend without refactoring the code. Thanks to #1844, a few ToJSON functions can be removed from the IR, and several constraints were simply redundant.

This PR borrows from similar work done as part of #1666.

## Note

To remove constraints more aggressively, I have explored the possibility of _removing Representable altogether_, in a [separate commit](https://github.com/hasura/graphql-engine-mono/compare/nicuveo/remove-extension-constraints..nicuveo/tentative-remove-representable). I am not convinced it's a good idea in terms of readability of the code, but it's a possibility.

Further work includes deciding what we want to do with `Show` and `ToTxt`; see #1747.

hasura/graphql-engine-mono#1843

GitOrigin-RevId: 337324a

## Description

When setting up a remote relationship to a remote schema, values coming from the left-hand side are given as _arguments_ to the targeted field of the remote schema. In turn, that means we need to adjust the arguments to that remote field; in the case of input objects, it means creating a brand new input object in which the relevant fields have been removed.

To both avoid conflicts, and be explicit, we give a pretty verbose name to such an input object: its original name, followed by "remote_rel", followed by the full name of the field (table name + relationship name). The bug there was introduced when working on extending remote relationships to other backends: we changed the code that translates the table name to a graphql identifier to be generic, and use the table's `ToTxt` instance instead. However, when a table is not in the default schema, the character used by that instance is `.`, which is not a valid GraphQL name.

This PR fixes it, by doing two things:
- it defines a safe function to translate LHS identifiers to graphql names (by replacing all invalid characters by `_`)
- it doesn't use `unsafeMkName` anymore, and checks at validation time that the type name is correct

## Further work

On this PR:
- [x] add a test
- [x] write a Changelog entry

Beyond this PR, we might want to:
- prioritize #1747
- analyze all calls to `unsafeMkName` and remove as many as possible

PR-URL: hasura/graphql-engine-mono#3363
GitOrigin-RevId: fe98eb1d34157b2c8323af453f5c369de616af38

<!-- The PR description should answer 2 important questions: -->

### What

Fixes some issues with the operator lookup for built-in NDC operators
(like `GreaterThan`, `LessThan` etc).
- Make error a user error, rather than internal
- Include more context in the error
- Also lookup operator in the OpenDD operators list if a mapping cannot
be found, as a missing mapping here means "the name is the same".

V3_GIT_ORIGIN_REV_ID: fef0003cb6d59610b6ec411ac21620b36af5a10b