[codex] Remove auto-compaction opt-out

[rhan-oai]

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 [codex] Add internal auto-compaction opt-out #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