Branch patterns with POSIX character classes like [[:digit:]] silently match the wrong branches #56

What's wrong

BranchPattern says it honours the same constructs as git's wildmatch for git branch --list (GitBranchStateCache/Refs/BranchPattern.cs:22-25). The #43 fix added ranges, sets, negation and escapes, but not POSIX character classes: [:alpha:], [:digit:], [:alnum:], [:upper:], [:lower:], [:space:], [:xdigit:], [:punct:] and the rest. git supports all of these.

In TryMatchBracket (BranchPattern.cs:190-214), the first ] after [[:digit: closes the bracket. The expression becomes the set { '[', ':', 'd', 'i', 'g', 't' } and the second ] is matched as a literal character.

Failure scenario

• Pattern: origin/release-[[:digit:]]*
• Branch: origin/release-1.0
• Here: no match, because 1 is not in the set.
• git: git branch --list 'release-[[:digit:]]*' lists the branch.

TryParse accepts the pattern, so /state and /branches return 200 with those branches missing. The client never hears about assets changed on them. The same pattern also wrongly matches a branch such as origin/release-d]x.

Suggested fix / acceptance criteria

• In TryMatchBracket, recognise [:name:] as a bracket member and test the character with the matching ASCII predicate (char.IsAsciiDigit, char.IsAsciiLetter, and so on). Keep the linear walk with no regular expressions.
• Reject an unknown class name such as [[:bogus:]] in TryParse with a 400, rather than letting it mismatch silently. git's wildmatch aborts the match in that case.
• Add tests for [[:digit:]], [[:digit:]], [[:alpha:]_-], a mix like [a[:digit:]], and the malformed [[:bogus:]].
• Update the remarks at BranchPattern.cs:22-25 to list the classes.