Fix: fail the license check when it inspected no files

[AmaadMartin]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):
   No existing issue; description below.
2. Or, if no issue exists, describe the change:

Problem: scripts/check_license.sh can report success without inspecting a
single file. It discards find's exit status, so a broken search prints zero
paths and the script still prints ✅ All files have the correct license header.
and exits 0. It also iterates the file list unquoted, so a path containing a
space splits into fragments, and perl -0777 -ne exits 0 on a file it cannot
open — an unlicensed file at such a path is accepted silently.

Solution: The script now writes NUL-terminated paths to a temp file, checks
find's exit status, rejects an empty result, and iterates a quoted bash array.
The original report suggested set -o pipefail, but the final design contains no
pipeline, so an explicit status check replaces it and gives the stronger
guarantee. The two new errors go to stderr and are worded differently, so a
failing CI log says which guard fired. Output and exit status are unchanged on
the clean-pass and the missing-header paths.

The script stays bash-3.2 and BSD-find compatible for the macos-latest
runner: while IFS= read -r -d '' instead of mapfile, and mktemp with an
explicit template. A trap 'rm -f "$FILE_LIST"' EXIT removes the temp file on
every exit path.

The temp file is what makes find's exit status readable. A pipe or a process
substitution hides it behind the read loop's own status, and that status is the
only way to catch a find that aborts partway: it prints the paths it reached,
so a partial scan otherwise reads as a full one. The empty-result guard does not
cover that case, because the list is not empty.

MISSING is a flag rather than a list of paths, because the loop already prints
each offending path and nothing reads the collection back.

Collision check: I listed all 593 open PRs on the fork and searched the whole
fork history for commits touching this file. No PR implements this fix. #594
edits the same find line for a separate cleanup, is not an ancestor of main,
and has no upstream PR, so this change is based on main and leaves that line's
prune clauses exactly as they are.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

New file tests/integration/scripts/check_license_test.ts runs the real script
against five scratch directories. Every fixture is written at runtime into an
fs.mkdtemp directory, because a committed unlicensed fixture would turn the
repository's own License Header Check red. The suite is skipped on Windows.

npx vitest run --project integration tests/integration/scripts/check_license_test.ts
Test Files  1 passed (1)      Tests  5 passed (5)

A .sh file is not instrumented by the v8 coverage provider and
coverage.include in vitest.config.ts lists only core/src, dev/src and
integrations/src, so this change does not move the coverage numbers. Each new
branch of the script gets a dedicated test case instead.

Proof that the tests can fail. I ran the suite against the unfixed script and
against three single-line mutations.

Mutation	Tests that fail	Reported failure
Whole script reverted to main	3 of 5 (empty result, space in path, find failure)	expected '' to contain 'no .js or .ts files were found'
Drop the if ! find ... status check	fails when find exits non-zero after printing a path	expected 'find: ./denied: Permission denied\n/u…' to contain 'failed to list source files'
Drop the [ "${#FILES[@]}" -eq 0 ] guard	fails when the search matches no .js or .ts file	expected '' to contain 'no .js or .ts files were found'
Change for FILE in "${FILES[@]}" to for FILE in ${FILES[@]}	checks a file whose directory name contains a space	expected '🔍 Checking for license headers...\n✅…' to contain '❌ Missing or invalid license header: …'
Drop the MISSING=1 assignment	2 of 5 (missing header, space in path)	expected '🔍 Checking…❌ Missing or invalid…' to contain 'Error: Some files are missing the required license header.'

The two pre-existing behaviours (clean pass, missing header) pass under every
mutation, which is what they are there to pin.

Manual End-to-End (E2E) Tests:

1. Happy path unchanged. bash scripts/check_license.sh from the repository
   root: exit 0 in 1.5s, empty stderr, and stdout byte-identical to the same
   command run from main (diff of the two captures is empty).
2. Broken search now fails. Delete the space before the escaped closing paren on
   the find line and run the copy: find's own diagnostic on stderr, then
   ❌ Error: failed to list source files; the license check did not run., exit
   1. The same mutation on main prints ✅ and exits 0.
3. Violation path unchanged. A scratch directory holding one unlicensed .ts
   file gives the ❌ Missing or invalid license header: line, the separator, the
   summary line, and exit 1.
4. No temp file leaks. ls "${TMPDIR:-/tmp}"/adk_license_files.* after all runs
   finds nothing.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.