Fix typing of Condition.each/for_each to return Condition[Collection] (#541)

[dweep-js]

Fixes #541.

Problem

Condition.each and Condition.for_each were typed to return
Condition[Iterable[E]]. Because Condition is invariant in its generic
parameter, PyCharm's type checker couldn't resolve
Condition[Iterable[Element]] against the Condition[Collection] bound
expected by a collection entity's .should() — even though Collection
is itself an Iterable[Element] at runtime. This produced a false-positive
type mismatch warning in PyCharm for code like:

ss('').should(have.text('').each)

Fix

Changed both return type annotations (for_each and the each property)
from Condition[Iterable[E]] to Condition[Collection], matching what
.should() actually expects on collection entities. Collection is
imported under TYPE_CHECKING to avoid a circular import with
entity.py.

Verification

• mypy selene/core/condition.py — no issues
• Confirmed via reveal_type() that Condition[Element].each now
  resolves to Condition[Collection] (previously
  Condition[Iterable[Element]])
• pytest tests/unit — all 62 tests pass
• black --check — clean

I don't have PyCharm available to visually confirm the IDE warning
disappears, but the mypy reveal_type output confirms the underlying
type resolution now matches what .should() expects. Would appreciate
a maintainer with PyCharm doing a final visual check on
ss('').should(have.text('').each).

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.23%. Comparing base (d0d7484) to head (03b0b27).

Files with missing lines	Patch %	Lines
selene/core/condition.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
- Coverage   69.27%   69.23%   -0.05%     
==========================================
  Files          50       50              
  Lines        3339     3341       +2     
  Branches      386      387       +1     
==========================================
  Hits         2313     2313              
- Misses        932      933       +1     
- Partials       94       95       +1     

Flag	Coverage Δ
regression	68.27% <60.00%> (-0.05%)	⬇️
unit	47.50% <40.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
selene/core/condition.py	68.33% <60.00%> (-0.77%)	⬇️

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