[ty] Validate forwarded tuple unions against unpacked callables

[charliermarsh]

Summary

Follow-up to #27517.

Previously, merging forwarded tuple alternatives could hide an incompatible unpacked argument or make an invalid overload appear applicable:

def callback(*args: *tuple[int, str]) -> None: ...

def forward(values: tuple[int, str] | tuple[int, int]) -> None:
    callback(*values)  # error: [invalid-argument-type]

We now retry binding against each expandable forwarded alternative, require every alternative to match a callable or overload, and preserve failing states when later expansion cannot prove the call is valid. Fixed arguments that can shift between tuple positions must satisfy every possible element type, and forwarded finite unions retain their minimum required length.

The existing expansion limit remains conservative; this change does not attempt to preserve successful calls after the limit is reached.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 96.97%. The percentage of expected errors that received a diagnostic held steady at 93.24%. The number of fully passing files held steady at 104/133.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Full report with detailed diff (timing results)