[Feature]: Replace fragile string-based reference rewriting

[samatstariongroup]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the EcoreNetto
• I have searched open and closed issues to ensure it has not already been reported

Description

• Summary: Cross-reference handling relies on string manipulation and hardcoded assumptions: the .ecore extension is hardcoded, %20 is the only URL escape decoded, and implicit #// references are rewritten by string concatenation. This is brittle for paths with other escaped characters or different extensions.
• Affected files: ECoreNetto/ModelElement/EObject.cs:392-416 (ProcessAttributeValue), ECoreNetto/ECoreParser.cs:85 (Replace("%20", " ")), ECoreNetto/Resource/Resource.cs:255 (.ecore check), ECoreNetto/ModelElement/NamedElement/EPackage.cs:212
• Proposed solution: Use Uri/Uri.UnescapeDataString for path decoding and proper URI fragment handling instead of ad-hoc string ops; avoid hardcoding the file extension where possible.
• Acceptance criteria:
  • Paths containing escaped characters beyond %20 load correctly.
  • Reference rewriting no longer depends on a hardcoded extension string in the hot path.
  • Tests cover an escaped-path and a cross-resource reference scenario.
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