fix(types): omit XValidation for required fields with references

[jonasz-lasut]

Description of your changes

When a field is marked as required but also has a reference configured, the field becomes satisfiable via *Ref/*Selector mechanisms. Explicitly set f.Required=false in NewReferenceField so no XValidation rule is emitted for such fields.

Fixes issue observed in crossplane-contrib/provider-upjet-gcp#952

I have:

• Read and followed Upjet's contribution process.
• Run make reviewable to ensure this PR is ready for review.
  - [ ] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Added unit test that validates this behavior

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Review profile: CHILL

Plan: Pro

Run ID: 1facd6c5-d79b-4c59-8efc-feb383fac6a1

📥 Commits

Reviewing files that changed from the base of the PR and between 993d58a and 3b9b01c.

📒 Files selected for processing (2)

• pkg/types/builder_test.go
• pkg/types/field.go

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📝 Walkthrough

Walkthrough

Referenced fields now clear their required state, and the build test adds a regression case confirming that a required referenced field does not emit XValidation.

Changes

Reference field required handling

Layer / File(s)	Summary
Reference field required flag pkg/types/field.go	NewReferenceField sets f.Required = false for referenced fields.
Build regression test pkg/types/builder_test.go	TestBuild adds a per-case setup hook and a regression case covering a required referenced field with no XValidation.

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 7

✅ Passed checks (7 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the core change: omitting XValidation for required fields that also have references.
Description check	✅ Passed	The description matches the code changes and explains the reference-driven Required=false behavior and test coverage.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Configuration Api Breaking Changes	✅ Passed	PASS: The PR only changes pkg/types files; no pkg/config/** files or public signatures were touched.
Generated Code Manual Edits	✅ Passed	No changed file matches zz_*.go; the PR only touches pkg/types/field.go and pkg/types/builder_test.go, so no generated-code manual edit is present.
Template Breaking Changes	✅ Passed	Only pkg/types/field.go and its test were changed; no pkg/controller/external*.go templates were modified.

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[ulucinar]

Thank you @jonasz-lasut, lgtm.

[github-actions]

Backport failed for release-1.11, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-1.11
git worktree add -d .worktree/backport-684-to-release-1.11 origin/release-1.11
cd .worktree/backport-684-to-release-1.11
git switch --create backport-684-to-release-1.11
git cherry-pick -x 3b9b01c64126c413d156c8867a2b0da399a5064b

[github-actions]

Backport failed for release-1.10, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-1.10
git worktree add -d .worktree/backport-684-to-release-1.10 origin/release-1.10
cd .worktree/backport-684-to-release-1.10
git switch --create backport-684-to-release-1.10
git cherry-pick -x 3b9b01c64126c413d156c8867a2b0da399a5064b