## Summary
Add a typed representation of function signatures (parameters and return
type) and infer it correctly from a function.
Convert existing usage of function return types to use the signature
representation.
This does not yet add inferred types for parameters within function body
scopes based on the annotations, but it should be easy to add as a next
step.
Part of #14161 and #13693.
## Test Plan
Added tests.
## Summary
- Get rid of `Symbol::unwrap_or` (unclear semantics, not needed anymore)
- Introduce `Type::call_dunder`
- Emit new diagnostic for possibly-unbound `__iter__` methods
- Better diagnostics for callables with possibly-unbound /
possibly-non-callable `__call__` methods
part of: #14022closes#14016
## Test Plan
- Updated test for iterables with possibly-unbound `__iter__` methods.
- New tests for callables
Minor cleanup and consistent formatting of the Markdown-based tests.
- Removed lots of unnecessary `a`, `b`, `c`, … variables.
- Moved test assertions (`# revealed:` comments) closer to the tested
object.
- Always separate `# revealed` and `# error` comments from the code by
two spaces, according to the discussion
[here](https://github.com/astral-sh/ruff/pull/13746/files#r1799385758).
This trades readability for consistency in some cases.
- Fixed some headings
## Summary
Porting infer tests to new markdown tests framework.
Link to the corresponding issue: #13696
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Co-authored-by: Carl Meyer <carl@astral.sh>
