[flutter_tools] Move plugin validation out of asserts (#191359)

## Description

Platform plugin `fromYaml` factories in `platform_plugins.dart`
(`AndroidPlugin`, `IOSPlugin`, `MacOSPlugin`, `WindowsPlugin`, and
`LinuxPlugin`) previously performed validation inside `assert`
statements. When Flutter tools runs in release mode (assertions
disabled), calling these factories directly could bypass validation and
return invalid plugin instances rather than surfacing a clean tool exit.

This PR:
1. Replaces `assert(validate(yaml))` in all platform plugin `fromYaml`
factories with explicit validation checks that call `throwToolExit`.
2. Includes the plugin name in the exit error message (e.g. `Invalid
"android" plugin specification for plugin "$name".`).
3. Adds automated unit tests in `plugins_test.dart` verifying that
`fromYaml` throws `ToolExit` when passed invalid YAML.

Fixes https://github.com/flutter/flutter/issues/67241

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to provide].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Flutter Style Guide]:
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[test-exempt]:
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