fix(search): emit the search_files truncation notice once and stop walking

[Tatlatat]

What

searchFiles pushes [… search truncated — refine pattern …] and returns
when the byte budget is exceeded. But return only unwinds the current
recursive walk frame — the parent loop keeps visiting sibling entries and
other directories. A later match while still over budget pushes the notice
again, so the output ends with duplicate truncation lines, and the walk
wastes I/O.

The sibling searchContent in the same file already guards this with a
truncated flag.

Repro

A workspace where matches span multiple directories and maxListBytes is small
enough to truncate after the first directory → output contains the truncation
notice twice.

Fix

Mirror searchContent: add a truncated flag, bail at the top of walk and
before each entry once it's set, and push the notice only on the first overflow.
Result: a single truncation notice and no wasted traversal.

Test

A search_files case asserting the truncation notice appears exactly once
across multiple directories. Verified it fails on the old code (two notices) and
passes on the fix.

Verification

npx vitest run tests/filesystem-tools.test.ts (123 passed); biome check
clean on the changed files.