[ty] Preserve all alternatives in bounded intersections (#27898)
Bounded intersection expansion could silently drop alternatives after
the first four members of a union. The first union is intentionally
exempt from the expansion budget, but ignoring a failed insertion did
not actually insert the omitted member. This let the helper return a
partial type as though it were exact, causing false-positive diagnostics
such as the regression exposed by #27664.
Apply the budget exemption inside candidate insertion. Preserve every
alternative of the first union, while continuing to return `None` if
later expansion exceeds the budget.
Fixes astral-sh/ty#4335.
## Test plan
- Unit tests cover late union members surviving an intersection in
either operand order, the single-union fast path, and genuine budget
exhaustion returning `None` instead of a partial result.
- Mdtests cover redundant upper bounds on five-member gradual unions,
including a reduced recursive-alias regression with `Divergent` members.
