feat: Support Await expressions

[gaoflow]

For reviewers

• I did not use AI
• I used AI and thoroughly reviewed every code/docs change

Description of the change

get_expression() currently supports every concrete Python expression node except
ast.Await, which raises KeyError. This adds the missing expression model, builder,
precedence handling, public export, and an AST-equivalence matrix covering Await in its
precedence-sensitive contexts.

Relevant resources

• https://docs.python.org/3/reference/expressions.html#await-expression
• https://docs.python.org/3/reference/expressions.html#operator-precedence

[pawamoy]

Looking good, thanks @gaoflow!