rv/rvgen: refactor automata.py to use iterator-based parsing

rv/rvgen: refactor automata.py to use iterator-based parsing

Refactor the DOT file parsing logic in automata.py to use Python's
iterator-based patterns instead of manual cursor indexing. The previous
implementation relied on while loops with explicit cursor management,
which made the code prone to off-by-one errors and would crash on
malformed input files containing empty lines.

The new implementation uses enumerate and itertools.islice to iterate
over lines, eliminating manual cursor tracking. Functions that search
for specific markers now use for loops with early returns and explicit
AutomataError exceptions for missing markers, rather than assuming the
markers exist. Additional bounds checking ensures that split line
arrays have sufficient elements before accessing specific indices,
preventing IndexError exceptions on malformed DOT files.

The matrix creation and event variable extraction methods now use
functional patterns with map combined with itertools.islice,
making the intent clearer while maintaining the same behavior. Minor
improvements include using extend instead of append in a loop, adding
empty file validation, and replacing enumerate with range where the
enumerated value was unused.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20260223162407.147003-12-wander@redhat.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>