testing: deduplicate integration stubs

[wprzytula]

This PR extracts one commit from #438 in order to merge it without further delay. It's authored by @dkropachev.
Original description:

The Rust integration layer already provides these test-only symbols when cpp_integration_testing is enabled. Drop the duplicate C++ stubs from testing_unimplemented.cpp so static integration builds link against a single definition.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• PR description sums up the changes and reasons why they should be introduced.
• [ ] I have implemented Rust unit tests for the features/changes introduced.
• [ ] I have enabled appropriate tests in Makefile in {SCYLLA,CASSANDRA}_(NO_VALGRIND_)TEST_FILTER.
• [ ] I added appropriate Fixes: annotations to PR description.

[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request removes 8 exported stub function implementations from src/testing_unimplemented.cpp. The removed functions—cass_aggregate_meta_name, cass_cluster_set_authenticator_callbacks, cass_function_meta_name, cass_keyspace_meta_aggregate_by_name, cass_keyspace_meta_function_by_name, cass_schema_meta_version, cass_statement_add_key_index, and cass_statement_set_keyspace—were all stub implementations that threw std::runtime_error("UNIMPLEMENTED ..."). The change streamlines the mock testing implementation to retain only actively-used stubs, reducing the exported symbol set by 8 functions while keeping the remaining stub functions intact.

Suggested labels

area/testing

Suggested reviewers

• Lorak-mmk

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description is complete, explaining the purpose of the PR and referencing the original source. Appropriate checklist items are marked as completed; items marked with strikethrough are intentionally excluded as not applicable.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.
Title check	✅ Passed	The title 'testing: deduplicate integration stubs' directly describes the main change: removing duplicate stub implementations from the testing_unimplemented.cpp file, which is the core purpose of the PR.

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[Copilot]

Pull request overview

This PR removes a handful of duplicate C API stub definitions from src/testing_unimplemented.cpp so that, when cpp_integration_testing is enabled, static integration-test builds don’t encounter multiple-definition linker errors and instead link against the Rust-provided test-only stub symbols.

Changes:

• Deleted several UNIMPLEMENTED C++ stub exports that are already provided by the Rust integration-testing layer when cpp_integration_testing is enabled.

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