🐛 Keep backslash escapes and entities in image alt text

[gyanu2507]

Fixes #445

Image alt text dropped backslash escapes and entities. ![a \* b](/u) rendered alt="a b" and ![C:\Python26](/u) rendered alt="C:ython26". Link text with the same content was fine.

The cause is in text_join. It turns text_special tokens into text and merges adjacent text, but only for the direct children of each inline token. An image holds its alt text as its own child token list, so the text_special tokens in there were left alone, and renderInlineAsText (which only emits text, nested images, line breaks and raw HTML) skipped them.

markdown-it fixed the same thing in 15.0.0 (markdown-it/markdown-it@974e2d7) by also joining the children of each child token. This ports that. The existing join loop moves into a _join_text_tokens helper that runs over the inline children and over each child's own children. Same one-level scope as upstream.

Tests

Added a case to tests/test_port/fixtures/issue-fixes.md with the four inputs from the upstream test plus ![C:\Python26](/u).

• pytest tests/test_port/test_fixtures.py -k issue_fixes: the new case fails without the change and passes with it.
• pytest tests/ (with the testing and linkify extras): 1033 passed.
• mdit-py-plugins test suite against this branch, as in the plugins tox env: 511 passed, same as on master.
• pre-commit run on the changed files (ruff, ruff format, mypy): passed.

Added a changelog entry under Unreleased.