Extract tool config into codex-tools (#16379)

## Why

`codex-core` already owns too much of the tool stack, and `AGENTS.md`
explicitly pushes us to move shared code out of `codex-core` instead of
letting it keep growing. This PR takes the next incremental step in
moving `core/src/tools` toward `codex-rs/tools` by extracting
low-coupling tool configuration and image-detail gating logic into
`codex-tools`.

That gives later extraction work a cleaner boundary to build on without
trying to move the entire tools subtree in one shot.

## What changed

- moved `ToolsConfig`, `ToolsConfigParams`, shell backend config, and
unified-exec session selection from `core/src/tools/spec.rs` into
`codex-tools`
- moved original image-detail gating and normalization into
`codex-tools`
- updated `codex-core` to consume the new `codex-tools` exports and pass
a rendered agent-type description instead of raw role config
- kept `codex-rs/tools/src/lib.rs` exports-only, with extracted unit
tests living in sibling `*_tests.rs` modules

## Testing

- `cargo test -p codex-tools`
- `cargo test -p codex-core --lib tools::spec::`

codex-rs/Cargo.lock

@@ -2627,11 +2627,15 @@ version = "0.0.0"
 dependencies = [
  "codex-app-server-protocol",
  "codex-code-mode",
+ "codex-features",
  "codex-protocol",
+ "codex-utils-absolute-path",
+ "codex-utils-pty",
  "pretty_assertions",
  "rmcp",
  "serde",
  "serde_json",
+ "tracing",
 ]
 
 [[package]]