fix: exclude custom_test.ts from coverage to prevent temp file issues

The integration tests in custom_test.ts create temporary files with
cache-busting fragments that cause coverage generation to fail when
the temporary files are cleaned up but still referenced in coverage.

This fix excludes the integration test from coverage while preserving
all test functionality.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: lambdalisue

deno.jsonc

@@ -7,7 +7,7 @@
   "tasks": {
     "check": "deno check ./**/*.ts",
     "test": "deno test -A --parallel --shuffle --doc",
-    "test:coverage": "deno task test --coverage=.coverage",
+    "test:coverage": "deno test -A --parallel --shuffle --doc --coverage=.coverage --ignore=denops/fall/custom_test.ts",
     "coverage": "deno coverage .coverage --exclude=testdata/",
     "update": "deno run --allow-env --allow-read --allow-write=. --allow-run=git,deno --allow-net=deno.land,jsr.io,registry.npmjs.org jsr:@molt/cli ./**/*.ts",
     "update:write": "deno task -q update --write",