[ty] Handle various invalid explicit specializations for ParamSpec (#21821)

## Summary

fixes: https://github.com/astral-sh/ty/issues/1788

## Test Plan

Add new mdtests.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ty_python_semantic/resources/mdtest/generics/legacy/paramspec.md

@@ -244,6 +244,7 @@ Explicit specialization of a generic class involving `ParamSpec` is done by prov
 of types, `...`, or another in-scope `ParamSpec`.
 
 ```py
+reveal_type(OnlyParamSpec[[]]().attr)  # revealed: () -> None
 reveal_type(OnlyParamSpec[[int, str]]().attr)  # revealed: (int, str, /) -> None
 reveal_type(OnlyParamSpec[...]().attr)  # revealed: (...) -> None
 
@@ -252,8 +253,28 @@ def func(c: Callable[P2, None]):
 
 # TODO: error: paramspec is unbound
 reveal_type(OnlyParamSpec[P2]().attr)  # revealed: (...) -> None
+
+# error: [invalid-type-arguments] "No type argument provided for required type variable `P1` of class `OnlyParamSpec`"
+reveal_type(OnlyParamSpec[()]().attr)  # revealed: (...) -> None
 ```
 
+An explicit tuple expression (unlike an implicit one that omits the parentheses) is also accepted
+when the `ParamSpec` is the only type variable. But, this isn't recommended is mainly a fallout of
+it having the same AST as the one without the parentheses. Both mypy and Pyright also allow this.
+
+```py
+reveal_type(OnlyParamSpec[(int, str)]().attr)  # revealed: (int, str, /) -> None
+```
+
+<!-- blacken-docs:off -->
+
+```py
+# error: [invalid-syntax]
+reveal_type(OnlyParamSpec[]().attr)  # revealed: (...) -> None
+```
+
+<!-- blacken-docs:on -->
+
 The square brackets can be omitted when `ParamSpec` is the only type variable
 
 ```py
@@ -269,6 +290,7 @@ reveal_type(OnlyParamSpec[int]().attr)  # revealed: (int, /) -> None
 But, they cannot be omitted when there are multiple type variables.
 
 ```py
+reveal_type(TypeVarAndParamSpec[int, []]().attr)  # revealed: () -> int
 reveal_type(TypeVarAndParamSpec[int, [int, str]]().attr)  # revealed: (int, str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, [str]]().attr)  # revealed: (str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, ...]().attr)  # revealed: (...) -> int
@@ -276,8 +298,12 @@ reveal_type(TypeVarAndParamSpec[int, ...]().attr)  # revealed: (...) -> int
 # TODO: We could still specialize for `T1` as the type is valid which would reveal `(...) -> int`
 # TODO: error: paramspec is unbound
 reveal_type(TypeVarAndParamSpec[int, P2]().attr)  # revealed: (...) -> Unknown
-# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be either a list of types, `ParamSpec`, `Concatenate`, or `...`"
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
 reveal_type(TypeVarAndParamSpec[int, int]().attr)  # revealed: (...) -> Unknown
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
+reveal_type(TypeVarAndParamSpec[int, ()]().attr)  # revealed: (...) -> Unknown
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
+reveal_type(TypeVarAndParamSpec[int, (int, str)]().attr)  # revealed: (...) -> Unknown
 ```
 
 Nor can they be omitted when there are more than one `ParamSpec`s.

crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md

@@ -228,6 +228,7 @@ Explicit specialization of a generic class involving `ParamSpec` is done by prov
 of types, `...`, or another in-scope `ParamSpec`.
 
 ```py
+reveal_type(OnlyParamSpec[[]]().attr)  # revealed: () -> None
 reveal_type(OnlyParamSpec[[int, str]]().attr)  # revealed: (int, str, /) -> None
 reveal_type(OnlyParamSpec[...]().attr)  # revealed: (...) -> None
 
@@ -238,8 +239,28 @@ P2 = ParamSpec("P2")
 
 # TODO: error: paramspec is unbound
 reveal_type(OnlyParamSpec[P2]().attr)  # revealed: (...) -> None
+
+# error: [invalid-type-arguments] "No type argument provided for required type variable `P1` of class `OnlyParamSpec`"
+reveal_type(OnlyParamSpec[()]().attr)  # revealed: (...) -> None
 ```
 
+An explicit tuple expression (unlike an implicit one that omits the parentheses) is also accepted
+when the `ParamSpec` is the only type variable. But, this isn't recommended is mainly a fallout of
+it having the same AST as the one without the parentheses. Both mypy and Pyright also allow this.
+
+```py
+reveal_type(OnlyParamSpec[(int, str)]().attr)  # revealed: (int, str, /) -> None
+```
+
+<!-- blacken-docs:off -->
+
+```py
+# error: [invalid-syntax]
+reveal_type(OnlyParamSpec[]().attr)  # revealed: (...) -> None
+```
+
+<!-- blacken-docs:on -->
+
 The square brackets can be omitted when `ParamSpec` is the only type variable
 
 ```py
@@ -255,14 +276,19 @@ reveal_type(OnlyParamSpec[int]().attr)  # revealed: (int, /) -> None
 But, they cannot be omitted when there are multiple type variables.
 
 ```py
+reveal_type(TypeVarAndParamSpec[int, []]().attr)  # revealed: () -> int
 reveal_type(TypeVarAndParamSpec[int, [int, str]]().attr)  # revealed: (int, str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, [str]]().attr)  # revealed: (str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, ...]().attr)  # revealed: (...) -> int
 
 # TODO: error: paramspec is unbound
 reveal_type(TypeVarAndParamSpec[int, P2]().attr)  # revealed: (...) -> Unknown
-# error: [invalid-type-arguments]
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
 reveal_type(TypeVarAndParamSpec[int, int]().attr)  # revealed: (...) -> Unknown
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
+reveal_type(TypeVarAndParamSpec[int, ()]().attr)  # revealed: (...) -> Unknown
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
+reveal_type(TypeVarAndParamSpec[int, (int, str)]().attr)  # revealed: (...) -> Unknown
 ```
 
 Nor can they be omitted when there are more than one `ParamSpec`.