fix: unused shared schema to serializer

[Eomm]

Closes #1491

This fix needs to be backported to v1.x

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

[mcollina]

Can you please add a bit of description of dontClearId, and why somebody should set it to true?

[Eomm]

That boolean needs because it resolve the shared schema when added without removing the $id that is needed by fastify-plugin.
If I remove that condition (only) this specific test fails:

fastify/test/decorator.test.js

Line 635 in cf7a096

test('a decorator should addSchema to all the encapsulated tree', t => {

because the $id would be removed from the object, than the same object reference would be given to children that throw a FST_ERR_SCH_MISSING_ID error.
I have tried to clone the object, but I get so many fails..

Only schemas.js should set it to true

---

Context:

When a shared schema is added, a pair of $id + JSON with $id is created

When a JSON schema is resolved by the validation.js during the startup, the JSON with $id is injected in the right place.
During the injection che $id field is deleted because, if a user use two or more time the same shared schema AJV will complain for "multiple equals $id".

If a user add a shared schema with a reference to another shared schema and doesn't use it, the shared shema will not resolved and the code break.

[mcollina]

Can you put some of this explanation in the code itself?

[Eomm]

Done

[mcollina]

LGTM, can you do a backport PR?

[github-actions]

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