feat: Add isAvailable() method to AbstractCommand

[patel-vansh]

Description
This PR adds a new method in AbstractCommand.php, isAvailable(). This method can be used to check in runtime if the command should be executed or not. Many times, we need to define environment specific commands, especially dev commands that shouldn't be callable in production or testing. In this case, one can use isAvailable() method to determine whether the command should execute or not.

The run() method will first call isAvailable() before any other lifecycle methods (like initialize()/interact()/execute(), etc.). The sole purpose of this method is to allow/restrict execution of commands based on runtime environment/config/state and NOT passed arguments or options, and that's why it doesn't accept any parameters.

If a caller calls a command that is not available for particular runtime, CommandNotAvailableException is thrown.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[Copilot]

Pull request overview

This PR introduces a runtime availability gate for modern Spark commands by adding an overridable isAvailable(): bool hook to CodeIgniter\CLI\AbstractCommand. The command runner now checks availability at the start of run() and throws a dedicated exception when a command is present but not allowed to execute in the current runtime context.

Changes:

• Added AbstractCommand::isAvailable() (default true) and enforced an availability check at the start of AbstractCommand::run().
• Introduced CommandNotAvailableException plus a new language line (Commands.notAvailable) for the error message.
• Updated user guide + changelog and added system tests/fixtures ensuring unavailable commands are still discoverable/listed/help-describable while being blocked from execution.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
system/CLI/AbstractCommand.php	Adds isAvailable() hook and throws CommandNotAvailableException early in run() when unavailable.
system/CLI/Exceptions/CommandNotAvailableException.php	New exception type for unavailable modern commands.
system/Language/en/Commands.php	Adds Commands.notAvailable language string for the exception message.
tests/_support/Commands/Modern/UnavailableFixtureCommand.php	New fixture modern command that can be toggled available/unavailable for testing.
tests/system/CLI/AbstractCommandTest.php	Adds coverage for throwing when unavailable and ensuring availability is checked before initialize/interact/execute.
tests/system/Commands/ListCommandsTest.php	Ensures unavailable commands remain visible in list output.
tests/system/Commands/HelpCommandTest.php	Ensures help can still describe an unavailable command.
user_guide_src/source/cli/cli_modern_commands.rst	Documents lifecycle update and new “Restricting Command Execution” section with example.
user_guide_src/source/cli/cli_modern_commands/013.php	New documentation example demonstrating isAvailable() usage.
user_guide_src/source/changelogs/v4.8.0.rst	Notes isAvailable() in the modern command hook list.

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