fix: blanket impl GraphQLFields for Option<T> for nullable queries

[flipbit03]

Summary

Two complementary fixes for nullable GraphQL query support:

1. Blanket impl (SDK): Add impl<T: GraphQLFields> GraphQLFields for Option<T> that delegates to the inner type's selection. This allows Option<T> to be used as the type parameter in generated query functions.

2. Codegen nullable detection: Modify codegen to detect nullable return types (Issue vs Issue!) and emit Result<Option<T>> instead of Result<T> for those queries. This encodes nullability in the function signature so callers don't need to guess.

Together, nullable queries like issueVcsBranchSearch now work end-to-end:

// Returns Result<Option<T>> — caller knows it might be null
let result = client.issue_vcs_branch_search::<Issue>(branch).await?;
match result {
    Some(issue) => println!("Found: {}", issue.identifier),
    None => println!("No issue for this branch"),
}

Also adds issueVcsBranchSearch to operations.toml as the first (and currently only) nullable non-connection query.

Closes #120

Test plan

• make check — clean
• make test — all offline tests pass (54 unit + 125 integration)
• cargo run -p lineark-codegen — regenerated, only issue_vcs_branch_search gets Result<Option<T>>
• 3 new unit tests in field_selection::tests:
  • option_delegates_selection_to_inner_type — Option<T>::selection() == T::selection()
  • option_preserves_full_type — compile-time proof that Option<T>::FullType == T::FullType
  • option_nullable_query_deserialization — full chain: GraphQLFields + serde null/object handling