[ty] Retain typing-only symbols in explicit completions

[charliermarsh]

Summary

We now exclude private type variables, parameter specifications, type-variable tuples, aliases, and @type_check_only definitions from completions, even though those names can be explicitly imported in typing-only contexts:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from package import _Alias, _T

This PR makes those definitions consistent with existing @type_check_only completion behavior: retain them as explicit import and attribute completions, mark them as typing-only, and rank them below runtime values.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 96.95%. The percentage of expected errors that received a diagnostic held steady at 92.39%. The number of fully passing files held steady at 102/133.

[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)

[AlexWaygood]

Thank you!