Remove duplicated fuzz suite trait residue

[chubes4]

Summary

• Remove the duplicated private fuzz-suite implementation from WP_Codebox_Abilities_Execution.
• Keep run_fuzz_suite() as the public ability guard/delegator to WP_Codebox_Fuzz_Suite_Runner.
• Keep descriptor-context helpers as thin delegators to the runner, including a public runtime-backed runner capability accessor on the runner.
• Update the public runtime abilities contract test to assert the runner owns the fuzz-suite implementation.

Tests

• php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-execution.php
• php -l packages/wordpress-plugin/src/class-wp-codebox-fuzz-suite-runner.php
• npm run test:php-fuzz-suite-runner
• npm run test:php-primitive-contract-parity
• npm run test:wordpress-plugin-public-runtime-abilities
• git diff --check

Residual risks

• Low: this is intended to be a mechanical ownership cleanup, but the public visibility change for fuzz_suite_runtime_backed_runner_capabilities_contract() slightly expands the runner class surface so the trait can avoid duplicating the contract array.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Scoped duplicate-code removal and verification