Ensure exhaustive switch statements

[bukzor]

PR #162 revealed that missing node type handlers in switch statements cause silent data loss - CDATA content was being dropped because CharDataNode wasn't handled. This PR adds defensive programming to prevent this entire class of bug.

Changes:

• Added exhaustive switch coverage for all xmlquery.NodeType values
• Added exhaustive switch coverage for xml.Token and html.TokenType
• Added exhaustive switch coverage for json.Token and json.Delim
• Added default cases with panic() to all exhaustive switches
• Added explicit documented defaults to intentionally non-exhaustive switches

This follows the Go stdlib pattern (runtime/panic.go, go/types) of using panic("unreachable") for "should be impossible" states. If future xmlquery versions add new node types, or if we overlook a handler, tests will fail immediately rather than silently corrupting output.

All switches now explicitly handle defaults:

• 8 exhaustive switches panic on unknown values
• 7 non-exhaustive switches have documented intentional behavior

Added TestExhaustiveNodeTypeHandling to verify all node types are processed without panicking. All existing tests pass.

This is a non-breaking change - no API modifications, only defensive additions to switch statements.

Related: #162

[codecov-commenter]

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Codecov Report

:x: Patch coverage is 52.38095% with 10 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 79.85%. Comparing base (ec5a59a) to head (6ddf893).

Files with missing lines	Patch %	Lines
internal/utils/utils.go	12.50%	7 Missing :warning:
internal/utils/jsonutil.go	83.33%	2 Missing :warning:
internal/utils/config.go	0.00%	1 Missing :warning:
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
- Coverage   80.57%   79.85%   -0.73%     
==========================================
  Files           5        5              
  Lines         690      710      +20     
==========================================
+ Hits          556      567      +11     
- Misses         92      101       +9     
  Partials       42       42              

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