refactor(manager-core): single source for executor-timeout default

Stage 6 review Obs 1b: the 300s default lived as a literal in two
places (DEFAULT_ASYNC_EXEC_TIMEOUT in dispatcher.rs and
DEFAULT_SAFE_VISIBILITY_VS_EXEC_BUDGET_SECS in triggers_api.rs). A
future change to the dispatcher would silently leave the
visibility-timeout warn threshold out of sync.

Extract a single pub const DEFAULT_ASYNC_EXEC_TIMEOUT_SECS: u32 = 300
in dispatcher.rs, derive the existing Duration constant from it, and
re-export it under the triggers_api name so the validator's
self-contained semantics are preserved at the call site. Tests still
inject the value explicitly via TEST_SAFE_LIMIT — no behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/manager-core/src/triggers_api.rs

@@ -41,10 +41,11 @@ use crate::trigger_repo::{
 const MIN_QUEUE_VISIBILITY_TIMEOUT_SECS: u32 = 30;
 
 /// Default soft-warning ceiling when `PICLOUD_DISPATCHER_ASYNC_EXEC_TIMEOUT_SEC`
-/// is unset — matches `DEFAULT_ASYNC_EXEC_TIMEOUT` in dispatcher.rs.
-/// Kept as a constant rather than imported so the validator stays
-/// self-contained for unit tests.
-const DEFAULT_SAFE_VISIBILITY_VS_EXEC_BUDGET_SECS: u32 = 300;
+/// is unset. Re-exported from the dispatcher so the single source of
+/// truth for the executor-budget default lives in one place — a future
+/// change to the dispatcher default shifts the visibility-timeout warn
+/// boundary in lockstep.
+use crate::dispatcher::DEFAULT_ASYNC_EXEC_TIMEOUT_SECS as DEFAULT_SAFE_VISIBILITY_VS_EXEC_BUDGET_SECS;
 
 /// Read the dispatcher's per-message executor budget at validation
 /// time so the warn-threshold tracks the live env-overridable value