Fix nested ref resolution for remote documents

[andreyyudin]

Nested $ref values inside remote documents were resolved using the root spec’s index/path instead of the referenced document’s context, which breaks relative refs inside remote files.

Changes

• Resolve nested refs using the referenced document’s index/path context when configured.
• Cache compiled JSONPath expressions to reduce repeated parse/alloc overhead during node lookups and tighten timeout handling.

[codecov]

Codecov Report

❌ Patch coverage is 83.78378% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.55%. Comparing base (b7fe7bb) to head (8ebc9da).

Files with missing lines	Patch %	Lines
datamodel/low/extraction_functions.go	58.33%	8 Missing and 2 partials ⚠️
index/resolver.go	93.93%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #505      +/-   ##
==========================================
- Coverage   99.60%   99.55%   -0.06%     
==========================================
  Files         189      189              
  Lines       22671    22721      +50     
==========================================
+ Hits        22582    22620      +38     
- Misses         56       65       +9     
- Partials       33       36       +3     

Flag	Coverage Δ
unittests	99.55% <83.78%> (-0.06%)	⬇️

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