refactor: remove obsolete length argument from reverse aggregation calls

[samukweku]

Summary

Every _*_rev_* wrapper in _agg_functions.py (and the size_rev
equivalents) still declared and forwarded a length: int parameter to
janitor_rs. On the Rust side that parameter was already obsolete
everywhere it survived -- either unused entirely (the plain starts/ends
shape derives its own bound from index.len()) or only ever feeding a
HashMap::with_capacity() preallocation hint, never correctness or
bounds-checking. This is the Python-side half of janitor-rs's own
"drop the unused length parameter from every reverse kernel" change.

What changed

• Removed length from every _*_rev_* wrapper's signature in
  _agg_functions.py, and from every call site (plus the now-dead
  length = ... local-variable computations that fed them) in
  _get_join_aggs.py. No accumulation logic or output changed.
• Added tests/functions/test_conditional_join_agg_boundary.py: focused,
  non-hypothesis tests that call each modified _agg_functions wrapper
  directly with a small hand-computed fixture -- one per reverse-aggregation
  shape (no_range, starts, ends, starts_matches, ends_matches,
  starts_ends, starts_ends_matches, positions, plus size's
  no-arr/booleans variants) -- and a parametrized regression guard
  asserting none of the 37 touched wrappers declares a length parameter
  any more. These are intentionally narrower and faster than the existing
  turtle-marked property tests in test_conditional_join.py (which
  already cover this boundary end-to-end and continue to pass unchanged);
  the point here is to catch a Python/Rust signature mismatch specifically
  -- the exact failure mode this whole change is about.

Testing

Verified against a janitor-rs build carrying the matching Rust-side
removal:

• The 49 new tests in test_conditional_join_agg_boundary.py: all pass.
• The full existing test_conditional_join.py suite: 242 passed, 1
  skipped. The only 36 failures were ModuleNotFoundError: No module named 'numba' from a missing optional dependency in the verification
  environment -- unrelated to this change (a separate, numba-accelerated
  code path that never touches _agg_functions.py/_get_join_aggs.py).

Before merging

This needs a janitor-rs release with the matching Rust-side length
removal -- until then, every touched call will raise
TypeError: unexpected keyword argument 'length' against the currently
pinned range (janitor-rs>=0.6.1,<0.7). Landing order should be
janitor-rs first, then this.

Scope

Rebased directly onto current dev (previously stacked on the unmerged
1649-simplify-conditional-join-rust-boundary branch, which this no
longer depends on) and expanded to cover every remaining shape that still
had length, not just the narrower no_range/size/prod_starts_matches
subset the branch covered before. #1684, #1686, and #1690 remain separate,
untouched PRs.

[github-actions]

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