Stop calling typeLoader for built-in scalar names

[spawnia]

Summary

Fixes #1874

Calling the typeLoader for built-in scalar names (String, Int, Float, Boolean, ID) breaks downstream consumers whose typeLoaders were not designed to handle these names.
This removes typeLoader-based scalar override support, keeping only types-based overrides.

• Remove the typeLoader scalar probe loop in getTypeMap() that iterated built-in scalar names
• Guard loadType() against built-in scalar names so the typeLoader is never called for them
• Convert all typeLoader-based scalar override tests to use types config
• Add regression test verifying typeLoader is not called for built-in scalar names

Test plan

• vendor/bin/phpunit tests/Type/ScalarOverridesTest.php — all 11 tests pass
• vendor/bin/phpunit — full suite green (1 pre-existing unrelated failure in EnumTypeTest)
• vendor/bin/phpstan — no errors

🤖 Generated with Claude Code

[ruudk]

How is this now going to be supported when using a type loader? With this Pr you can only do it when using a types map?

[ruudk]

I'd rather have an option that enables scalars via type loader. Make the default behavior in a new version. But it allows people to opt-in when using a type loader.

Also, I have the feeling that people that want custom scalar types, often also have a type loader. So making this only apply to people without a type loader does not make much sense to me.

[ruudk]

So basically I would vote for #1882

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.50.

Benchmark suite	Current: 89cd752	Previous: 317a71e	Ratio
DeferredBench::benchManyNestedDeferreds	21.75 ms	12.163 ms	1.79

This comment was automatically generated by workflow using github-action-benchmark.

[spawnia]

typeLoader users can still override built-in scalars — they just use types for it instead of the typeLoader itself:

$schema = new Schema([
    'query' => $queryType,
    'typeLoader' => $myTypeLoader,
    'types' => [$uppercaseString],
]);

I added a test that proves both mechanisms work together: the typeLoader resolves custom types (like User) while types overrides the built-in String scalar, and both are applied correctly.

Also added documentation for this in the scalars docs and updated the types option description in the schema definition docs.

[ruudk]

Oké great let's try it out!