[ty] Improve diagnostics if the user attempts to import a stdlib module that does not exist on their configured Python version (#18403)

crates/ty_python_semantic/resources/mdtest/import/basic.md

@@ -176,3 +176,32 @@ emitted for the `import from` statement:
 # error: [unresolved-import]
 from does_not_exist import foo, bar, baz
 ```
+
+## Attempting to import a stdlib module that's not yet been added
+
+<!-- snapshot-diagnostics -->
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+import tomllib  # error: [unresolved-import]
+from string.templatelib import Template  # error: [unresolved-import]
+from importlib.resources import abc  # error: [unresolved-import]
+```
+
+## Attempting to import a stdlib module that was previously removed
+
+<!-- snapshot-diagnostics -->
+
+```toml
+[environment]
+python-version = "3.13"
+```
+
+```py
+import aifc  # error: [unresolved-import]
+from distutils import sysconfig  # error: [unresolved-import]
+```

crates/ty_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Attempting_to_import…_(2fcfcf567587a056).snap

@@ -0,0 +1,65 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Attempting to import a stdlib module that's not yet been added
+mdtest path: crates/ty_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | import tomllib  # error: [unresolved-import]
+2 | from string.templatelib import Template  # error: [unresolved-import]
+3 | from importlib.resources import abc  # error: [unresolved-import]
+```
+
+# Diagnostics
+
+```
+error[unresolved-import]: Cannot resolve imported module `tomllib`
+ --> src/mdtest_snippet.py:1:8
+  |
+1 | import tomllib  # error: [unresolved-import]
+  |        ^^^^^^^
+2 | from string.templatelib import Template  # error: [unresolved-import]
+3 | from importlib.resources import abc  # error: [unresolved-import]
+  |
+info: The stdlib module `tomllib` is only available on Python 3.11+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `string.templatelib`
+ --> src/mdtest_snippet.py:2:6
+  |
+1 | import tomllib  # error: [unresolved-import]
+2 | from string.templatelib import Template  # error: [unresolved-import]
+  |      ^^^^^^^^^^^^^^^^^^
+3 | from importlib.resources import abc  # error: [unresolved-import]
+  |
+info: The stdlib module `string.templatelib` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Module `importlib.resources` has no member `abc`
+ --> src/mdtest_snippet.py:3:33
+  |
+1 | import tomllib  # error: [unresolved-import]
+2 | from string.templatelib import Template  # error: [unresolved-import]
+3 | from importlib.resources import abc  # error: [unresolved-import]
+  |                                 ^^^
+  |
+info: The stdlib module `importlib.resources` only has a `abc` submodule on Python 3.11+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Attempting_to_import…_(c14954eefd15211f).snap

@@ -0,0 +1,47 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Attempting to import a stdlib module that was previously removed
+mdtest path: crates/ty_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | import aifc  # error: [unresolved-import]
+2 | from distutils import sysconfig  # error: [unresolved-import]
+```
+
+# Diagnostics
+
+```
+error[unresolved-import]: Cannot resolve imported module `aifc`
+ --> src/mdtest_snippet.py:1:8
+  |
+1 | import aifc  # error: [unresolved-import]
+  |        ^^^^
+2 | from distutils import sysconfig  # error: [unresolved-import]
+  |
+info: The stdlib module `aifc` is only available on Python <=3.12
+info: Python 3.13 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `distutils`
+ --> src/mdtest_snippet.py:2:6
+  |
+1 | import aifc  # error: [unresolved-import]
+2 | from distutils import sysconfig  # error: [unresolved-import]
+  |      ^^^^^^^^^
+  |
+info: The stdlib module `distutils` is only available on Python <=3.11
+info: Python 3.13 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```