Input validators crash fix (#4868)

* Fix crash when using validate_on with list

* Ensure tests cover crash scenario of validate_on not being a set

* Update CHANGELOG

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Added `tooltip` to Binding https://github.com/Textualize/textual/pull/4859
 
+### Fixed
+
+- Fix crash when `validate_on` value isn't a set https://github.com/Textualize/textual/pull/4868
+
 ## [0.76.0]
 
 ### Changed

src/textual/widgets/_input.py

@@ -2,7 +2,7 @@
 
 import re
 from dataclasses import dataclass
-from typing import TYPE_CHECKING, ClassVar, Iterable, cast
+from typing import TYPE_CHECKING, ClassVar, Iterable
 
 from rich.cells import cell_len, get_character_cell_size
 from rich.console import Console, ConsoleOptions, RenderableType
@@ -301,7 +301,7 @@ def __init__(
             self.validators = list(validators)
 
         self.validate_on: set[str] = (
-            (_POSSIBLE_VALIDATE_ON_VALUES & cast("set[str]", validate_on))
+            (_POSSIBLE_VALIDATE_ON_VALUES & set(validate_on))
             if validate_on is not None
             else _POSSIBLE_VALIDATE_ON_VALUES
         )

tests/input/test_input_validation.py

@@ -100,10 +100,11 @@ async def test_on_blur_triggers_validation():
     "validate_on",
     [
         set(),
+        [],
         {"blur"},
-        {"submitted"},
-        {"blur", "submitted"},
-        {"fried", "garbage"},
+        ["submitted"],
+        ["blur", "submitted"],
+        ["fried", "garbage"],
     ],
 )
 async def test_validation_on_changed_should_not_happen(validate_on):