fix: error when invalidating a nonexistent environment

[NishchayMahor]

What

sqlmesh invalidate ENVIRONMENT reports success even when the environment doesn't exist:

$ sqlmesh invalidate doesnotexist
Environment 'doesnotexist' invalidated.

That's misleading — a mistyped environment name looks like it worked. This checks that the environment exists first and raises a clear error with a nonzero exit code otherwise:

$ sqlmesh invalidate doesnotexist
Error: Environment 'doesnotexist' does not exist.
$ echo $?
1

How

Context.invalidate_environment now looks the environment up via state_sync.get_environment before invalidating, and raises SQLMeshError when it's absent (the CLI's @error_handler maps that to exit code 1). The message and exception match the existing "environment not found" patterns already used elsewhere in context.py.

Testing

• New unit test test_invalidate_environment_nonexistent_raises (fails without the fix).
• Existing test_invalidate_environment_{sync,no_sync} and the test_invalidating_environment integration test still pass.
• Verified end to end on a fresh sqlmesh init duckdb project: nonexistent env → Error: Environment '...' does not exist. with exit code 1.

Fixes #5621