Skip parent relationships without an indexable symbol

[mindaugasrukas]

Propagate the symbol writer's success status when saving parent relationships, and omit relationships whose target symbol is skipped. Keep the existing nonempty-symbol assertion for successful writes.

Motivation

saveSymbolString can legitimately return a skip result when a parent has no indexable source location. The callback used by saveParentRelationships currently discards that result, then attempts to add a relationship with an empty target and triggers !rel.symbol().empty().

The included regression reproduces the assertion with:

# typed: true

describe "example" do
  before { @value = 1 }
end

@value

Return status.ok() through the callback and stop constructing that relationship when it returns false.

Test plan

• Confirmed the regression triggers the empty-relationship-symbol assertion before the fix.
• Added a snapshot test for the reproduction above.
• Ran ./bazel test --config=dev --jobs=6 //test/scip with this patch alone: all 60 tests passed, including existing inheritance and mixin snapshots.
• Built //main:scip-ruby with --config=dev.

Validation ran on macOS ARM64 using the macOS 14.4 SDK with the repository's LLVM 15 toolchain; the SDK selection was local to the validation checkout and is not part of this patch.