8.0.x - make configurable: allow-bean-definition-overriding / allow-circular-references

[codeconsole]

Summary

We shouldn't force bad practices and start providing an exit strategy

WARNING: Bean overriding will be deprecated in a future release.

GrailsApp and the unit-test harness GrailsApplicationBuilder hardcoded
allowBeanDefinitionOverriding(true) and allowCircularReferences(true). This
silently overrode any value an application set through the standard Spring Boot
properties spring.main.allow-bean-definition-overriding and
spring.main.allow-circular-references — so there was no way to turn either off.

Both now resolve those properties from the configured environment and default
to true to preserve Grails' historical behavior (a plain Spring Boot app
defaults both to false). Applications can now opt into stricter startup
semantics:

spring:
  main:
    allow-bean-definition-overriding: false
    allow-circular-references: false

Changes

• grails-core — GrailsApp.createApplicationContext() resolves both flags
  from the configured environment (defaulting to true).
• grails-testing-support-core — GrailsApplicationBuilder.createMainContext()
  resolves the same properties, read after configuration data is loaded and
  before refresh().
• docs — new "Bean Definition Overriding and Circular References" section in
  the Application class configuration guide.
• tests — GrailsAppContextOverridingSpec (grails-core) and
  GrailsApplicationBuilderContextOverridingSpec (grails-databinding).

Notes

• The default is intentionally true. Grails' own bootstrap relies on bean
  definition overriding (the harness re-registers infrastructure beans such as
  internalConfigurationAnnotationProcessor). The test suite documents that
  disabling overriding makes a Grails context fail fast with
  BeanDefinitionOverrideException — which is exactly why the default differs
  from Spring Boot's.
• GrailsApplicationBuilder lives in grails-testing-support-core, but its full
  bootstrap requires the servlet API, so its behavioral test lives in
  grails-databinding (which has the servlet API on its test classpath and
  already exercises the builder).

Verification

• ./gradlew :grails-core:test :grails-databinding:test :grails-testing-support-core:test :grails-fields:test --rerun-tasks — green.
• checkstyleMain/codenarcMain + checkstyleTest/codenarcTest on the
  changed modules — no violations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.0000%. Comparing base (87c000a) to head (9482237).
⚠️ Report is 10 commits behind head on 8.0.x.

Additional details and impacted files

@@      Coverage Diff       @@
##   8.0.x   #15765   +/-   ##
==============================
==============================

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[testlens-app]

✅ All tests passed ✅

⚠️ TestLens detected flakiness ⚠️

Test Summary

CI / Functional Tests (Java 21, indy=true) > :grails-test-examples-app1:integrationTest

Test	Runs
AsyncPromiseSpec > multi-stage process reports all stages	❌ ✅

🏷️ Commit: 9482237
▶️ Tests: 4498 executed
⚪️ Checks: 44/44 completed

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