Fix integer assignment in exploded text fragments

[FanWu-ai]

explode_text_fragments() returns a list whose integer assignments should replace an existing fragment. Currently, assigning to -1 inserts before the last fragment, and out-of-range integer assignments silently insert instead of raising IndexError.

For example:

fragments = explode_text_fragments([('', 'abc')])
fragments[-1] = ('bold', 'Z')
# Expected: [('', 'a'), ('', 'b'), ('bold', 'Z')]
# Before:   [('', 'a'), ('', 'b'), ('bold', 'Z'), ('', 'c')]

Normalize negative integer indices and check bounds before converting to a one-element slice. Explicit slice assignment and automatic expansion of replacement fragments are unchanged. Adds 23 regression cases for valid positive/negative indices, empty/single/multicharacter replacements, invalid indices, empty lists and custom __index__ objects.

Validation:

• Complete test suite: 179 passed on Python 3.12.14 and 3.13.5.
• The unchanged implementation fails 8 cases in the regression module; the fixed module passes all 25 cases.
• Ruff lint/format, typos and strict mypy for Linux/Windows/macOS passed.
• A separate deterministic check passed 1,034 cases for integer-index parity, replacement metadata and unchanged slice behavior.

This fixes public-helper behavior; a specific built-in UI failure has not been established. Other Python versions in the CI matrix were not run locally. This branch is based directly on main and is independent of #2120.

OpenAI Codex assisted with investigation, implementation, testing, automated review and this description. The exact model identifier was not available; no human-review claim is made.