[ty] Preserve minimum lengths of unpacked tuple unions (#29331)
Reject calls where every member of an unpacked tuple union supplies too
many arguments once later positional arguments are included:
```python
def f(a: int) -> None: ...
def g(args: tuple[int] | tuple[int, int]) -> None:
f(*args, 1) # too-many-positional-arguments
```
Combining unequal tuple shapes previously discarded their minimum
lengths, allowing argument matching to treat `args` as empty.
`TupleSpecBuilder::union` now preserves the smaller minimum length when
combining shapes, so `Type::iterate` retains that guarantee for all its
consumers. This also preserves required elements when unpacking tuple
unions into tuple literals or inferring a `TypeVarTuple`.
Addresses the [minimum-length review
comment](https://github.com/astral-sh/ruff/pull/29293#discussion_r4248949931)
on #29293.
## Ecosystem report
Analyzed head:
[`1b5eb5424c`](https://github.com/astral-sh/ruff/commit/1b5eb5424c7fad6edf6994d73a36691f358a97e2).
[Full
report](https://github.com/astral-sh/ruff/pull/29331#issuecomment-6150237369).
The five changes are understood and acceptable for this fix: two more
precise Bokeh messages, two rewrites of existing Meson cyclic-inference
warnings, and one duplicate SymPy argument error. No newly rejected
calls were found. The duplicate reflects pre-existing reporting behavior
for nonempty tuples and can be addressed independently.
