test(core): move prompt debug coverage to integration suite (#18916)

## 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:

```text
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`

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/18916).
* #18913
* __->__ #18916

codex-rs/core/tests/suite/mod.rs

@@ -67,6 +67,7 @@ mod personality;
 mod personality_migration;
 mod plugins;
 mod prompt_caching;
+mod prompt_debug_tests;
 mod quota_exceeded;
 mod realtime_conversation;
 mod remote_env;