core: cut codex-core compile time 48% with native async SessionTask

[bolinfest]

Why

This continues the compile-time cleanup from #16630. SessionTask implementations are monomorphized, but Session stores the task behind a dyn boundary so it can drive and abort heterogenous turn tasks uniformly. That means we can move the #[async_trait] expansion off the implementation trait, keep a small boxed adapter only at the storage boundary, and preserve the existing task lifecycle semantics while reducing the amount of generated async-trait glue in codex-core.

One measurement caveat showed up while exploring this: a warm incremental benchmark based on touch core/src/tasks/mod.rs && cargo check -p codex-core --lib was basically flat, but that was the wrong benchmark for this change. Using package-clean codex-core rebuilds, like #16630, shows the real win.

Relevant pre-change code:

• SessionTask with #[async_trait]
• RunningTask storing Arc<dyn SessionTask>

What changed

• Switched SessionTask::{run, abort} to native RPITIT futures with explicit Send bounds.
• Added a private AnySessionTask adapter that boxes those futures only at the Arc<dyn ...> storage boundary.
• Updated RunningTask to store Arc<dyn AnySessionTask> and removed #[async_trait] from the concrete task impls plus test-only SessionTask impls.

Timing

Benchmarked package-clean codex-core rebuilds with dependencies left warm:

cargo check -p codex-core --lib >/dev/null
cargo clean -p codex-core >/dev/null
/usr/bin/time -p cargo +nightly rustc -p codex-core --lib -- \
  -Z time-passes \
  -Z time-passes-format=json >/dev/null

revision	rustc total	process real	generate_crate_metadata	MIR_borrow_checking	monomorphization_collector_graph_walk
parent 3c7f013f9735	67.21s	67.71s	24.61s	23.43s	22.43s
this PR 2cafd783ac22	35.08s	35.60s	8.01s	7.25s	7.15s
delta	-47.8%	-47.4%	-67.5%	-69.1%	-68.1%

For completeness, the warm touched-file benchmark stayed flat (1.96s parent vs 1.97s this PR), which is why that benchmark should not be used to evaluate this refactor.

Verification

• Ran cargo test -p codex-core; this change compiled and task-related tests passed before hitting the same unrelated 5 config::tests::*guardian* failures already present on the parent stack.