[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/comparison/tuples.md

@@ -33,7 +33,9 @@ reveal_type(a >= b)  # revealed: Literal[False]
 
 Even when tuples have different lengths, comparisons should be handled appropriately.
 
-```py path=different_length.py
+`different_length.py`:
+
+```py
 a = (1, 2, 3)
 b = (1, 2, 3, 4)
 
@@ -102,7 +104,9 @@ reveal_type(a >= b)  # revealed: bool
 However, if the lexicographic comparison completes without reaching a point where str and int are
 compared, Python will still produce a result based on the prior elements.
 
-```py path=short_circuit.py
+`short_circuit.py`:
+
+```py
 a = (1, 2)
 b = (999999, "hello")