Propagate cache key and service tiers in compact (#21249)

## Why

`/responses/compact` should preserve the request-affinity fields that
apply to the active auth mode. ChatGPT-auth compact requests need the
effective `service_tier`, and compact requests for every auth mode need
the stable `prompt_cache_key`, so compaction does not quietly lose
routing or cache behavior that normal sampling already has.

This follows the request-parity direction from #20719, but keeps the net
change focused on the compact payload fields needed here.

## What changed

- Add `service_tier` and `prompt_cache_key` to the compact endpoint
input payload.
- Build the remote compact payload from the existing responses request
builder output so `Fast` still maps to `priority` when compact sends a
service tier.
- Pass the turn service tier into remote compaction, but only include it
in compact payloads for ChatGPT-backed auth.
- Keep `prompt_cache_key` on compact payloads for all auth modes.
- Add request-body diff snapshot coverage in
`core/tests/suite/compact_remote.rs` for:
- API-key auth reusing `prompt_cache_key` while omitting `service_tier`
even when `Fast` is configured.
  - ChatGPT auth reusing both `service_tier` and `prompt_cache_key`.
- Drive the snapshot coverage through five varied turns: plain text,
multi-part text, tool-call continuation, image+text input, local-shell
continuation, and final-turn reasoning output.

## Verification

- Added insta snapshots for compact request-body parity against the last
normal `/responses` request after five varied turns.
- Not run locally per repo guidance; relying on GitHub CI for test
execution.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-api/src/common.rs

@@ -32,6 +32,10 @@ pub struct CompactionInput<'a> {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub reasoning: Option<Reasoning>,
     #[serde(skip_serializing_if = "Option::is_none")]
+    pub service_tier: Option<&'a str>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub prompt_cache_key: Option<&'a str>,
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub text: Option<TextControls>,
 }