Fix Table column widths being truncated for the first and last columns

[yyccPhil]

Overview: What does this pull request change?

This fixes the first and last columns of a Table (and its subclasses MathTable, IntegerTable, and DecimalTable) being truncated when explicit col_widths are passed through arrange_in_grid_config. Grid lines and cell borders now follow each column's arranged slot instead of the bounding box of its contents, so the requested column widths are honored for every column.

Closes #3446

Motivation and Explanation: Why and how do your changes improve the library?

When col_widths makes a column's slot wider than its contents, arrange_in_grid centers the (narrower) content inside the slot. However, Table._add_vertical_lines() and Table.get_cell() anchored the outer grid lines and cell borders to the content extent (get_columns()[i].get_left()/get_right() ± half h_buff) rather than to the slot. The outer columns were therefore drawn narrower than requested, and because the anchor depended on the content width, the truncation was asymmetric (a column with narrow contents was clipped more).

Reproduction from the issue, rendered at width 10 with col_widths=[3] * 6:

• Before: per-column widths ≈ [1.00, 1.84, 1.85, 1.85, 1.85, 1.60] — first and last columns visibly clipped.
• After: [1.667] * 6 — all columns equal, col_widths honored.

Before / after

Before:

After:

The change adds a small helper, Table._get_column_x_edges(col_index), which returns the left/right x-coordinates of a column's arranged slot:

• When no width is set for a column, it returns the column's content edges — i.e. exactly the previous behavior — so default tables are unaffected. (I verified numerically that the vertical-line positions and get_cell() corners are unchanged to within 1e-9 for a default table.)
• When a fixed width is set, it returns the slot edges and respects col_alignments ("l", "c", "r").

_add_vertical_lines() (both outer and inner lines) and get_cell() now use this helper.

Links to added or changed documentation pages

No documentation pages are changed by this PR.

Further Information and Comments

• Added a regression test, test_table_col_widths_are_honored, in tests/module/mobject/test_table.py, modeled on the existing regression tests there. It fails on main and passes with this change.
• Scoping question for reviewers: when row/column labels are present they shift column indexing relative to col_widths, so the helper intentionally keeps the previous content-based behavior for labelled tables to avoid changing their output. I'm happy to extend the fix to labelled tables as well if you'd prefer — just let me know which you'd like.

This is my first contribution to Manim, so any feedback is very welcome!

Acceptance criteria

• Tests added for new/changed behavior (test_table_col_widths_are_honored)
• All tests passing (existing tests/module/mobject/test_table.py suite + the new test)
• Follows the project style guide (ruff check and ruff format --check pass)
• No breaking changes introduced (verified the default, no-col_widths path is byte-for-byte unchanged)
• Documentation updated (if applicable) — n/a, no documentation pages affected

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested