test: organize shared test types and narrow test files

[coderabbitai]

Summary

Improve the organization of the C++ test suite so that test code is easier for human readers to navigate and understand.

Required changes

• Move test-only types and utilities that are used by more than one test file into shared test support headers.
• Split broad test files into smaller test files with one narrow focus per file.
• Keep each test file focused on one API area or behavior.

Rationale

The resource-limiting tests introduce several test-only resource types and related helpers. As the test suite grows, local copies and large multi-purpose test files make it harder to find relevant coverage and understand the test intent.

Affected areas

• C++ test sources under test/
• Shared C++ test support headers, where appropriate
• Resource-limiting test coverage added in PR feat(core): add resource-limited execution to graph nodes #850

Acceptance criteria

• Test-only types used by multiple test files are defined in shared test support headers.
• Each affected test file has a narrow, clear responsibility.
• Existing test coverage remains present after the reorganization.
• The test build configuration includes each resulting test source.

References

• Pull request: feat(core): add resource-limited execution to graph nodes #850
• Review comment: feat(core): add resource-limited execution to graph nodes #850 (comment)
• Requested by: @knoepfel