fix(rss): handle self-closing XML elements correctly

[bug-ops]

Summary

• Fix RSS parsing to properly handle self-closing (empty) XML elements
• Root cause: skip_element() was called for empty elements expecting a closing tag
• This consumed subsequent XML events and broke parsing of <item> elements

Changes

• Add is_empty flag propagation through parse_channel and parse_item functions
• Skip skip_element() calls when is_empty=true at both channel and item level
• Fix enclosure element handling (enclosures are always self-closing)
• Populate feed.feed.image from itunes:image for empty elements

Test Plan

• Added 16 comprehensive tests covering various empty element scenarios
• All 616 tests pass
• Clippy clean with -D warnings
• Code formatted with cargo +nightly fmt
• cargo deny check passes

Closes #45

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.15924% with 39 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/feedparser-rs-core/src/parser/rss.rs	75.15%	39 Missing ⚠️

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
- Coverage   91.84%   91.68%   -0.16%     
==========================================
  Files          34       34              
  Lines        7758     7816      +58     
==========================================
+ Hits         7125     7166      +41     
- Misses        633      650      +17     

Flag	Coverage Δ
rust-core	91.68% <75.15%> (-0.16%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/feedparser-rs-core/src/parser/rss.rs	94.11% <75.15%> (-0.74%)	⬇️

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