fix: move inline codex-rs/core unit tests into sibling files

[bolinfest]

Why

PR #13783 moved the codex.rs unit tests into codex_tests.rs. This applies the same extraction pattern across the rest of codex-rs/core so the production modules stay focused on runtime code instead of large inline test blocks.

Keeping the tests in sibling files also makes follow-up edits easier to review because product changes no longer have to share a file with hundreds or thousands of lines of test scaffolding.

What changed

• replaced each inline mod tests { ... } in codex-rs/core/src/** with a path-based module declaration
• moved each extracted unit test module into a sibling *_tests.rs file, using mod_tests.rs for mod.rs modules
• preserved the existing cfg(...) guards and module-local structure so the refactor remains structural rather than behavioral

Testing

• cargo test -p codex-core --lib (1653 passed; 0 failed; 5 ignored)
• just fix -p codex-core
• cargo fmt --check
• cargo shear