fix(decorations): validate botline for on_win

Problem:
Many decoration providers (treesitter injection highlighting, semantic
token highlighting, inlay hint) rely on the correctness of the `botline`
argument of `on_win` callback. However, `botline` can be smaller than
the actual line number of the last displayed line if some lines are
folded. In such cases, some decorations will be missing in the lines not
covered by `botline`.

Solution:
Validate `botline` when invoking `on_win`.

NOTE:
It seems that the old code was deliberately avoiding this presumably due
to performance reasons. However, I haven't experienced noticeable lag
after this change, and I believe the cost of botline computation would
be much smaller than the cost of decoration providers.

src/nvim/api/extmark.c

@@ -1009,10 +1009,8 @@ void nvim_buf_clear_namespace(Buffer buffer, Integer ns_id, Integer line_start,
 ///             - on_buf: called for each buffer being redrawn (before
 ///                 window callbacks)
 ///                 ["buf", bufnr, tick]
-///             - on_win: called when starting to redraw a
-///                 specific window. botline_guess is an approximation
-///                 that does not exceed the last line number.
-///                 ["win", winid, bufnr, topline, botline_guess]
+///             - on_win: called when starting to redraw a specific window.
+///                 ["win", winid, bufnr, topline, botline]
 ///             - on_line: called for each buffer line being redrawn.
 ///                 (The interaction with fold lines is subject to change)
 ///                 ["win", winid, bufnr, row]