test: add boundary tests for text(max_nb_chars)

[saimoon504]

Related to #2389

Brief summary: adds three tests to tests/providers/test_lorem.py
pinning down the boundary behavior of text(max_nb_chars).

What was wrong

The existing lorem tests exercise text() per locale but do not test
its boundary behavior. While testing Faker for a university course we
noticed this gap when investigating #2389.

How this fixes it

Adds a TestTextBounds class with three tests:

• test_text_respects_upper_bound - output never exceeds max_nb_chars,
  checked around the internal mode switches at 25 and 100 characters
• test_text_minimum_length_raises - values below 5 raise ValueError
  (enforced in the code but previously untested)
• test_text_ext_word_list_only - with ext_word_list, every word in the
  output comes from the given list

All three pass locally: 3 passed in 0.67s on Python 3.10.12.