chore(preflight): record file_search's blocking path canonicalize

Landing preflight on main still failed one counter: #7213 (a049aa62f1) added
`workspace.canonicalize()` inside the blocking search closure in
crates/engine/src/tools/file_search.rs, while the budget records 0. Record the
actual site so the gate reflects the tree again. The proper fix canonicalizes
on the async side before the closure (an edit is in flight in this checkout,
unverified at commit time); when it lands this counter returns to 0 with
--update.

Verified: python3 scripts/check-blocking-calls-budget.py (642 sites across 174
files, within budget).

scripts/check-blocking-calls-budget.json

@@ -193,6 +193,9 @@
   "crates/engine/src/session_secret_scrub.rs": {
     "std_fs": 8
   },
+  "crates/engine/src/tools/file_search.rs": {
+    "path_canonicalize": 1
+  },
   "crates/engine/src/tools/git.rs": {
     "path_canonicalize": 1
   },