Preserve explicit table lines outside the natural edge span (fixes #1335)

[Cyberfilo]

What

When page.extract_table(...) is called with vertical_strategy="explicit" (or horizontal_strategy="explicit"), the user-supplied lines define the column / row boundaries. Edges of the opposite orientation are still derived from text clustering or detected page lines, and those are only as wide as the underlying objects.

words_to_edges_h (and the lines variants for horizontal edges) span just min_x0..max_x1 of the rows they detect. edges_to_intersections then keeps only the vertical edges whose x0 falls inside [h.x0 - x_tol, h.x1 + x_tol]. When an explicit vertical line sits outside that span — common when the leftmost or rightmost column is narrow, right-aligned, or contains only short / multi-line text — the intersection is rejected and the entire column collapses into its neighbour (or disappears entirely if it is the only one beyond the span).

Fix

In TableFinder.get_edges(), after the base + explicit edges are concatenated, extend each horizontal edge's x0 / x1 to cover the x-range of any explicit vertical lines (and symmetrically extend vertical edges' top / bottom to cover any explicit horizontals).

Explicit edges already span the full page when supplied as floats, and span their declared bbox when supplied as dicts — so this only ever widens edges derived from words or page lines, never narrows or moves anything. Tables that do not use explicit lines are untouched.

Repro / test

Added test_issue_1335_explicit_outside_text_span in tests/test_table.py. It constructs a three-row, three-column layout with a narrow last column (text ends ~130 px before the rightmost explicit vertical) and asserts that:

• all four explicit vertical positions produce intersections;
• the resulting table contains rows with all three column-cells present.

Without the patch, only the inner two explicit verticals produce intersections and the table collapses to a single column. Running the new test against develop:

$ pytest tests/test_table.py::Test::test_issue_1335_explicit_outside_text_span
... FAILED  (assert [200.0, 350.0] == [50, 200, 350, 500])

With the patch:

$ pytest tests/test_table.py::Test::test_issue_1335_explicit_outside_text_span
... PASSED

Verification

• make tests — 165 passed (the pre-existing test_repair.py::test_from_issue_932 failure on systems without Ghostscript is unchanged).
• make lint — clean (flake8, mypy --strict, black --check, isort --check).
• Added entry under ## Unreleased in CHANGELOG.md and added myself to the contributors list per CONTRIBUTING.md.

Fixes #1335.

[Cyberfilo]

@jsvine @samkit-jain please review