Add Windows unified exec yield floor (#27086)

## Why

The Windows `unified_exec` experiment regressed at the turn level in a
way that points to premature backgrounding / extra command cycles rather
than individual responses getting heavier:

- `codex_local_tool_calls_per_turn` was up about 20.7%.
- `codex_local_blended_tokens_per_turn` was up about 4.1%, and
`codex_local_output_tokens_per_turn` was up about 4.0%.
- `codex_local_response_latency_per_turn` was up about 8.3%.
- The primary activity metrics also moved down: `codex_turns` about
-6.6%, `codex_dau` about -1.0%, and `codex_local_hourly_active_users`
about -3.0%.

At the same time, the per-response metrics moved in the other direction:
blended tokens per response, output tokens per response, and latency per
response were all lower in test. That suggests the bad turn-level shape
is largely about extra tool/model cycles, not each response being slower
or more expensive on its own.

Local Windows benchmarking showed the likely mechanism: shell-wrapped
commands pay a large PowerShell startup/teardown tax before the actual
command has much time to run. In the benchmark, the PowerShell wrapper
added roughly 0.7-1.0s versus direct exec:

- Windows PowerShell: about 740ms p50 / 800ms p90 overhead versus direct
exec.
- PowerShell 7 (`pwsh`): about 930ms p50 / 980ms p90 overhead versus
direct exec.

The model commonly asks for a 1s initial yield. On Windows, that can
spend nearly the whole window waiting on PowerShell machinery, so
otherwise-short commands are more likely to return as background
sessions and require follow-up polling/tool calls.

This is intentionally a temporary unlock. It gives Windows closer to the
same useful post-shell command window as other platforms while we work
on reducing the PowerShell tax directly, for example with persistent
PowerShell workers or conservative direct-exec paths for commands that
do not need shell semantics.

## What changed

- Adds a Windows-only 2s floor to `unified_exec`'s initial
`yield_time_ms` clamp.
- Keeps larger model-requested waits unchanged, including the existing
10s default.
- Keeps the existing 30s max clamp.
- Leaves non-Windows behavior unchanged.
- Adds platform-gated tests for both the Windows floor and the
non-Windows clamp behavior.

## Verification

- `just test -p codex-core unified_exec`

codex-rs/core/src/unified_exec/mod.rs

@@ -62,6 +62,7 @@ pub(crate) use process::SpawnLifecycleHandle;
 pub(crate) use process::UnifiedExecProcess;
 
 pub(crate) const MIN_YIELD_TIME_MS: u64 = 250;
+pub(crate) const WINDOWS_INITIAL_EXEC_YIELD_TIME_FLOOR_MS: u64 = 2_000;
 // Minimum yield time for an empty `write_stdin`.
 pub(crate) const MIN_EMPTY_YIELD_TIME_MS: u64 = 5_000;
 pub(crate) const MAX_YIELD_TIME_MS: u64 = 30_000;
@@ -165,6 +166,11 @@ struct ProcessEntry {
 }
 
 pub(crate) fn clamp_yield_time(yield_time_ms: u64) -> u64 {
+    let yield_time_ms = if cfg!(windows) {
+        yield_time_ms.max(WINDOWS_INITIAL_EXEC_YIELD_TIME_FLOOR_MS)
+    } else {
+        yield_time_ms
+    };
     yield_time_ms.clamp(MIN_YIELD_TIME_MS, MAX_YIELD_TIME_MS)
 }