[red-knot] Enforce specifying paths for mdtest code blocks in a separate preceding line (#15890)

## Summary

Resolves #15695, rework of #15704.

This change modifies the Mdtests framework so that:

* Paths must now be specified in a separate preceding line:

	`````markdown
	`a.py`:

	```py
	x = 1
	```
	`````

If the path of a file conflicts with its `lang`, an error will be
thrown.

* Configs are no longer accepted. The pattern still take them into
account, however, to avoid "Unterminated code block" errors.
* Unnamed files are now assigned unique, `lang`-respecting paths
automatically.

Additionally, all legacy usages have been updated.

## Test Plan

Unit tests and Markdown tests.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/expression/boolean.md

@@ -54,7 +54,9 @@ reveal_type("x" or "y" and "")  # revealed: Literal["x"]
 
 ## Evaluates to builtin
 
-```py path=a.py
+`a.py`:
+
+```py
 redefined_builtin_bool: type[bool] = bool
 
 def my_bool(x) -> bool: