[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/goto_declaration.rs

@@ -18,12 +18,9 @@ pub fn goto_declaration(
     let module = parsed_module(db, file).load(db);
     let goto_target = find_goto_target(&module, offset)?;
 
-    let declaration_targets = goto_target.get_definition_targets(
-        file,
-        db,
-        None,
-        ImportAliasResolution::ResolveAliases,
-    )?;
+    let declaration_targets = goto_target
+        .get_definition_targets(file, db, ImportAliasResolution::ResolveAliases)?
+        .declaration_targets(db)?;
 
     Some(RangedValue {
         range: FileRange::new(file, goto_target.range()),