test(core): move prompt debug coverage to integration suite

[bolinfest]

Why

build_prompt_input now initializes ExecServerRuntimePaths, which requires a configured Codex executable path. The previous inline unit test in core/src/prompt_debug.rs built a bare test_config() and then failed before it could assert anything useful:

Codex executable path is not configured

This coverage is also integration-shaped: it drives the public build_prompt_input entry point through config, thread, and session setup rather than testing a small internal helper in isolation.

Bazel CI did not catch this earlier because the affected test was behind the same wrapped Rust unit-test path fixed by #18913. Before that launcher/sharding fix, the outer workspace_root_test changed the working directory for Insta compatibility while the inner rules_rust sharding wrapper still expected its runfiles working directory. In practice, Bazel could report success without executing the Rust test cases in that shard. Once #18913 makes the wrapper run the Rust test binary directly and shard with libtest arguments, this stale unit test actually runs and exposes the missing codex_self_exe setup.

What Changed

• Moved build_prompt_input_includes_context_and_user_message out of core/src/prompt_debug.rs.
• Added core/tests/suite/prompt_debug_tests.rs and registered it from core/tests/suite/mod.rs.
• Builds the test config with ConfigBuilder and provides codex_self_exe using the current test executable, matching the runtime-path invariant required by prompt debug setup.
• Preserves the existing assertions that the generated prompt input includes both the debug user message and project-specific user instructions.

Verification

• cargo test -p codex-core --test all prompt_debug_tests::build_prompt_input_includes_context_and_user_message
• bazel test //codex-rs/core:core-all-test --test_arg=prompt_debug_tests::build_prompt_input_includes_context_and_user_message --test_output=errors

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Stack created with Sapling. Best reviewed with ReviewStack.

• bazel: run wrapped Rust unit test shards #18913
• -> test(core): move prompt debug coverage to integration suite #18916

[starr-openai]

thanks!