`enable_typename_discriminated_unions` feature bug - abstract inline fragment selections incorrectly merged onto concrete union arms

I am trying out enable_typename_discriminated_unions. Thank you, @morrys and relay team for pushing this feature forward!

I found a bug. Sharing some repros. I would like to hear your thoughts on what we think the expected output should be.

Bug: When typegen handles a fragment on an abstract type that mixes abstract and concrete inline fragments, it builds incorrect discriminated union arms:

Case 1 :

Fields from an abstract spread (e.g. ... on Named { name }) are merged onto concrete arms that do not implement that abstract type (e.g. Page).

Input

fragment AbstractConcreteMixedInline on Actor {
  __typename
  id
  ... on Named { name }       # abstract type -  only User implements Named (not Page)
  ... on User { firstName }
  ... on Page { subscribeStatus }
}

Output - name incorrectly on Page

| { __typename: "Page"; id: string; name?: string; subscribeStatus: string }
| { __typename: "User"; id: string; name?: string; firstName: string }
| { __typename: "%other" }


Expected Output

{ __typename: "User"; id: string; name?: string; firstName?: string } | { __typename: "Page"; id: string; subscribeStatus?: string } // no name | { __typename: "%other"; id: string }

Reference [test](https://github.com/Lalitha-Iyer/relay/blob/d8acd4ccf603a27f4ab4c90f02b1a61f15f1745f/compiler/crates/relay-typegen/tests/generate_typescript/fixtures/abstract-concrete-mixed-inline.expected)

### Case 2: 
Concrete arms implied only by an abstract spread can be missing (e.g., no User arm when ... on Named is selected but ... on User is not).

Input

fragment AbstractNamedWithConcretePage on Actor { __typename id ... on Named { name } ... on Page { subscribeStatus } }

Actual Output - User arm missing; name on Page

{ __typename: "Page"; id: string; name?: string; subscribeStatus: string } { __typename: "%other" }

Expected Output 

{ __typename: "User"; id: string; name?: string } # this can be a union eg: __typename: User | FooNamedType | { __typename: "Page"; id: string; subscribeStatus?: string } // no name | { __typename: "%other"; id: string }

Reference [test](https://github.com/Lalitha-Iyer/relay/blob/d8acd4ccf603a27f4ab4c90f02b1a61f15f1745f/compiler/crates/relay-typegen/tests/generate_typescript/fixtures/abstract-named-with-concrete-page.expected)