[ty] render docstrings in hover (#19882)

This PR has several components:

* Introduce a Docstring String wrapper type that has render_plaintext
and render_markdown methods, to force docstring handlers to pick a
rendering format
* Implement [PEP-257](https://peps.python.org/pep-0257/) docstring
trimming for it
* The markdown rendering just renders the content in a plaintext
codeblock for now (followup work)
* Introduce a `DefinitionsOrTargets` type representing the partial
evaluation of `GotoTarget::get_definition_targets` to ideally stop at
getting `ResolvedDefinitions`
* Add `declaration_targets`, `definition_targets`, and `docstring`
methods to `DefinitionsOrTargets` for the 3 usecases we have for this
operation
* `docstring` is of course the key addition here, it uses the same basic
logic that `signature_help` was using: first check the goto-declaration
for docstrings, then check the goto-definition for docstrings.
* Refactor `signature_help` to use the new APIs instead of implementing
it itself
* Not fixed in this PR: an issue I found where `signature_help` will
erroneously cache docs between functions that have the same type (hover
docs don't have this bug)
* A handful of new tests and additions to tests to add docstrings in
various places and see which get caught


Examples of it working with stdlib, third party, and local definitions:
<img width="597" height="120" alt="Screenshot 2025-08-12 at 2 13 55 PM"
src="https://github.com/user-attachments/assets/eae54efd-882e-4b50-b5b4-721595224232"
/>
<img width="598" height="281" alt="Screenshot 2025-08-12 at 2 14 06 PM"
src="https://github.com/user-attachments/assets/5c9740d5-a06b-4c22-9349-da6eb9a9ba5a"
/>
<img width="327" height="180" alt="Screenshot 2025-08-12 at 2 14 18 PM"
src="https://github.com/user-attachments/assets/3b5647b9-2cdd-4c5b-bb7d-da23bff1bcb5"
/>

Notably modules don't work yet (followup work):
<img width="224" height="83" alt="Screenshot 2025-08-12 at 2 14 37 PM"
src="https://github.com/user-attachments/assets/7e9dcb70-a10e-46d9-a85c-9fe52c3b7e7b"
/>

Notably we don't show docs for an item if you hover its actual
definition (followup work, but also, not the most important):
<img width="324" height="69" alt="Screenshot 2025-08-12 at 2 16 54 PM"
src="https://github.com/user-attachments/assets/d4ddcdd8-c3fc-4120-ac93-cefdf57933b4"
/>

crates/ty_ide/src/references.rs

@@ -47,12 +47,9 @@ pub(crate) fn references(
     // Get the definitions for the symbol at the cursor position
 
     // When finding references, do not resolve any local aliases.
-    let target_definitions_nav = goto_target.get_definition_targets(
-        file,
-        db,
-        None,
-        ImportAliasResolution::PreserveAliases,
-    )?;
+    let target_definitions_nav = goto_target
+        .get_definition_targets(file, db, ImportAliasResolution::PreserveAliases)?
+        .definition_targets(db)?;
     let target_definitions: Vec<NavigationTarget> = target_definitions_nav.into_iter().collect();
 
     // Extract the target text from the goto target for fast comparison
@@ -287,12 +284,10 @@ impl LocalReferencesFinder<'_> {
 
         if let Some(goto_target) = GotoTarget::from_covering_node(covering_node, offset) {
             // Get the definitions for this goto target
-            if let Some(current_definitions_nav) = goto_target.get_definition_targets(
-                self.file,
-                self.db,
-                None,
-                ImportAliasResolution::PreserveAliases,
-            ) {
+            if let Some(current_definitions_nav) = goto_target
+                .get_definition_targets(self.file, self.db, ImportAliasResolution::PreserveAliases)
+                .and_then(|definitions| definitions.declaration_targets(self.db))
+            {
                 let current_definitions: Vec<NavigationTarget> =
                     current_definitions_nav.into_iter().collect();
                 // Check if any of the current definitions match our target definitions