Fix parsing of RGB codes that use colon delimiters

[alcarney]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

There are two ways to "spell" the escape code sequence to set a truecolor value. One way uses semicolon delimiters, while the other uses colon delimiters

However, when parsing escape codes using Text.from_ansi, rich only recognises codes that use semicolon delimiters

❯ uv run --with rich python
Python 3.13.5 (main, Jun 26 2025, 21:20:04) [Clang 20.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from rich.text import Text

>>> Text.from_ansi('\x1b[38;2;255;0;0mred\x1b[0m text')
<text 'red text' [Span(0, 3, Style(color=Color('#ff0000', ColorType.TRUECOLOR, triplet=ColorTriplet(red=255, green=0, blue=0))))] ''>

>>> Text.from_ansi('\x1b[38:2::255:0:0mred\x1b[0m text')
<text 'red text' [] ''>

This PR tweaks the AnsiDecoder so that it converts colons to semicolons, allowing both style of codes to be parsed