[codex] Remove auto-compaction opt-out (#29815)

## Summary

- remove the default-on `auto_compaction` feature flag and generated
config schema entries
- restore unconditional pre-turn, model-switch/hash, and mid-turn
automatic compaction
- expose `new_context` whenever token-budget tooling is enabled
- remove the disabled-auto-compaction integration coverage introduced by
#28260

## Motivation

Roll back the internal auto-compaction escape hatch added in #28260.
Automatic compaction should no longer be suppressible with `--disable
auto_compaction`; existing manual `/compact` behavior remains unchanged.

## Testing

- `just write-config-schema`
- `just test -p codex-features` — 53 passed
- `just test -p codex-core 'suite::compact::'` — 36 passed
- `just test -p codex-core
suite::token_budget::new_context_tool_starts_new_window_before_follow_up`
— 1 passed
- `just fix -p codex-core -p codex-features`
- `just fmt`
- `just test -p codex-core` — 2,778 passed, 59 failed, 16 skipped;
failures were outside the changed compaction paths and were dominated by
missing first-party test binaries and shell-snapshot timeouts

codex-rs/core/config.schema.json

@@ -433,9 +433,6 @@
             "auth_elicitation": {
               "type": "boolean"
             },
-            "auto_compaction": {
-              "type": "boolean"
-            },
             "browser_use": {
               "type": "boolean"
             },
@@ -4744,9 +4741,6 @@
         "auth_elicitation": {
           "type": "boolean"
         },
-        "auto_compaction": {
-          "type": "boolean"
-        },
         "browser_use": {
           "type": "boolean"
         },