fix: guard composed ref walker against odd-length mapping content (#607)

[SAY-5]

BundleBytesComposed panics with index out of range when a $ref resolves to an array with an odd number of elements (for example a root-level tags list that arrives as a mapping-tagged node during composition). walkAndRewriteRefs steps through a MappingNode's content two at a time assuming key/value pairs and reads Content[i+1], which is out of bounds when the length is odd. This adds a bounds check: if the final key has no paired value, the dangling node is walked directly instead of indexing past the end. Even-length arrays are unaffected. Repro and fix verified against the reproduction in #607, and the full bundler suite plus go test ./... pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (0837c9b) to head (e266f89).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #608   +/-   ##
=======================================
  Coverage   99.78%   99.78%           
=======================================
  Files         283      283           
  Lines       34456    34459    +3     
=======================================
+ Hits        34382    34385    +3     
  Misses         46       46           
  Partials       28       28           

Flag	Coverage Δ
unittests	99.78% <100.00%> (+<0.01%)	⬆️

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