fix(crawler): escape + character in MIME type regex patterns

[marevol]

Summary

This PR extends the regex pattern fix from #3032 to escape the + character in additional MIME type patterns:

• crawler/rule.xml: Escape + in application/xhtml+xml and application/rdf+xml patterns for both webFileRule and fsFileRule
• fess_thumbnail.xml: Fix double-escaped \\+ to single-escaped \+ for image/svg+xml pattern

Changes Made

• Escape + character in application/xhtml+xml regex pattern (webFileRule, fsFileRule)
• Escape + character in application/rdf+xml regex pattern (webFileRule, fsFileRule)
• Fix over-escaped image/svg\\+xml to image/svg\+xml in thumbnail generator config
• Add comprehensive unit tests for MIME type pattern matching in CrawlerRuleMimeTypePatternTest.java
• Add additional tests for SVG/XHTML/RDF MIME type handling in BaseThumbnailGeneratorTest.java

Technical Details

The + character has special meaning in regex (matches "one or more" of the previous character). Without escaping, patterns like application/xhtml+xml would not match the literal MIME type string because the regex engine interprets l+ as "one or more 'l' characters" rather than the literal sequence l+.

Testing

• Added CrawlerRuleMimeTypePatternTest.java with 18 test cases covering:
  • Pattern matching for all MIME types in webFileRule and fsFileRule
  • Demonstration of the bug (unescaped + fails to match)
  • Verification that escaped patterns work correctly
• Extended BaseThumbnailGeneratorTest.java with tests for isTarget() method with SVG, XHTML, and RDF MIME types

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