[ty] Fix panic when inferring subscript assignment multiple times
When subscript assignments are validated, the closure that infers the
assigned type may be called multiple times with different type contexts
(e.g., for union narrowing or when validating against multiple overloads).
Previously, this would cause a panic due to the assertion in
`store_expression_type_impl` that fails when the same expression's type
is stored twice with `MultiInferenceState::Panic` (the default state).
This fix follows the same pattern used by attribute assignment validation:
track whether this is the first inference, and if not, temporarily set
`MultiInferenceState::Overwrite` to allow overwriting previously inferred
types without panicking.
Fixes astral-sh/ty#2503
