fix(core) Preserve base_instructions in SessionMeta (#9427)

## Summary
This PR consolidates base_instructions onto SessionMeta /
SessionConfiguration, so we ensure `base_instructions` is set once per
session and should be (mostly) immutable, unless:
- overridden by config on resume / fork
- sub-agent tasks, like review or collab


In a future PR, we should convert all references to `base_instructions`
to consistently used the typed struct, so it's less likely that we put
other strings there. See #9423. However, this PR is already quite
complex, so I'm deferring that to a follow-up.

## Testing
- [x] Added a resume test to assert that instructions are preserved. In
particular, `resume_switches_models_preserves_base_instructions` fails
against main.

Existing test coverage thats assert base instructions are preserved
across multiple requests in a session:
- Manual compact keeps baseline instructions:
core/tests/suite/compact.rs:199
- Auto-compact keeps baseline instructions:
core/tests/suite/compact.rs:1142
- Prompt caching reuses the same instructions across two requests:
core/tests/suite/prompt_caching.rs:150 and
core/tests/suite/prompt_caching.rs:157
- Prompt caching with explicit expected string across two requests:
core/tests/suite/prompt_caching.rs:213 and
core/tests/suite/prompt_caching.rs:222
- Resume with model switch keeps original instructions:
core/tests/suite/resume.rs:136
- Compact/resume/fork uses request 0 instructions for later expected
payloads: core/tests/suite/compact_resume_fork.rs:215

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

@@ -388,9 +388,10 @@ async fn remote_models_apply_remote_base_instructions() -> Result<()> {
 
     wait_for_event(&codex, |event| matches!(event, EventMsg::TurnComplete(_))).await;
 
+    let base_model_info = models_manager.get_model_info("gpt-5.1", &config).await;
     let body = response_mock.single_request().body_json();
     let instructions = body["instructions"].as_str().unwrap();
-    assert_eq!(instructions, remote_base);
+    assert_eq!(instructions, base_model_info.base_instructions);
 
     Ok(())
 }