[hooks_runner] Don't time out on missing output.json

[dcharkes]

If build hooks didn't output an output.json the build hooks runner would think it failed to acquire a file lock.

This PR fixes this by checking that the file system exceptions from locks are not mixed up with other file system exceptions, and adds an integration test.

I presume everyone is using the build and link methods from package:hooks, so I think it's unlikely anyone runs into this.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:code_assets	1.0.0	already published at pub.dev
package:data_assets	0.19.6	already published at pub.dev
package:ffi	2.1.5-wip	WIP (no publish necessary)
package:hooks	1.0.0	already published at pub.dev
package:hooks_runner	1.0.1	ready to publish	hooks_runner-v1.0.1
package:native_toolchain_c	0.17.4	already published at pub.dev
package:record_use	0.5.0-wip	WIP (no publish necessary)
package:swift2objc	0.1.0	already published at pub.dev
package:swiftgen	0.1.1	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/objective_c/example/command_line/lib/main.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

This check can be disabled by tagging the PR with skip-license-check.

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources

This check can be disabled by tagging the PR with skip-leaking-check.

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
hooks_runner	None	1.0.0	1.0.1	1.0.0	✔️

This check can be disabled by tagging the PR with skip-breaking-check.

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

This check can be disabled by tagging the PR with skip-changelog-check.

[coveralls]

coverage: 81.969%. remained the same
when pulling 8aade87 on missing-output-hang
into 59a519f on main.

[goderbauer]

LGTM