INTEGRATION_schema_test fails for new versions of xmllint

[scpeters]

Environment

• OS Version: 26.04
• Source or binary build? source build of 76713ad from ci.yml: test on Ubuntu 26.04 Resolute #1655

Description

• Expected behavior: INTEGRATION_schema_test should pass
• Actual behavior: INTEGRATION_schema_test fails

Steps to reproduce

1. Install xmllint from libxml2-utils on Ubuntu or libxml2 on brew, with a version at least 2.15.1. On Ubuntu 26.04 and homebrew, the default version is higher than this
2. Compile sdformat and run ctest -VV -R INTEGRATION_schema_test

Output

from CI in #1655: https://github.com/gazebosim/sdformat/actions/runs/25358850325/job/74487225142?pr=1655

[ RUN      ] SDFSchemaGenerator.TestDoublePendulum
  I/O warning : failed to load "http://sdformat.org/schemas/types.xsd": No such file or directory
  /github/workspace/build/sdf/1.12/root.xsd:8: element include: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}include': Failed to load the document 'http://sdformat.org/schemas/types.xsd' for inclusion.
  WXS schema /github/workspace/build/sdf/1.12/root.xsd failed to compile
  CMD[xmllint --noout --schema /github/workspace/build/sdf/1.12/root.xsd /github/workspace/test/integration/model/double_pendulum.sdf]
  /github/workspace/test/integration/schema_test.cc:40: Failure
  Failed
  Fail in parsing the model
  [  FAILED  ] SDFSchemaGenerator.TestDoublePendulum (2 ms)
  [ RUN      ] SDFSchemaGenerator.TestPR2Model
  I/O warning : failed to load "http://sdformat.org/schemas/types.xsd": No such file or directory
  /github/workspace/build/sdf/1.12/root.xsd:8: element include: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}include': Failed to load the document 'http://sdformat.org/schemas/types.xsd' for inclusion.
  WXS schema /github/workspace/build/sdf/1.12/root.xsd failed to compile
  CMD[xmllint --noout --schema /github/workspace/build/sdf/1.12/root.xsd /github/workspace/test/integration/model/pr2.sdf]
  /github/workspace/test/integration/schema_test.cc:40: Failure
  Failed
  Fail in parsing the model
  [  FAILED  ] SDFSchemaGenerator.TestPR2Model (1 ms)
  [ RUN      ] SDFSchemaGenerator.TestTurtleBotModel
  I/O warning : failed to load "http://sdformat.org/schemas/types.xsd": No such file or directory
  /github/workspace/build/sdf/1.12/root.xsd:8: element include: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}include': Failed to load the document 'http://sdformat.org/schemas/types.xsd' for inclusion.
  WXS schema /github/workspace/build/sdf/1.12/root.xsd failed to compile
  CMD[xmllint --noout --schema /github/workspace/build/sdf/1.12/root.xsd /github/workspace/test/integration/model/turtlebot.sdf]
  /github/workspace/test/integration/schema_test.cc:40: Failure
  Failed
  Fail in parsing the model
  [  FAILED  ] SDFSchemaGenerator.TestTurtleBotModel (1 ms)

[scpeters]

see comments in #1655 (comment) for a partial bisecting of libxml2 versions that narrows the breaking change between 2.13.8 and 2.15.1

[scpeters]

for now we should detect the version of xmllint and disable this test if the version is too new

[scpeters]

to get early warning of this type of breakage, we could also update our homebrew CI to use xmllint from homebrew-core rather than the old macOS system version