Extract MergeSourceSpans helper and deduplicate condition branches in tag helper resolution phase

[Copilot]

Why

DefaultTagHelperResolutionPhase and its partial-class companions (ComponentTagHelperResolver, LegacyTagHelperResolver) each had several places that manually constructed a merged SourceSpan by inlining the same arithmetic — computing an end absolute index, subtracting the start, etc. This duplication was:

• Error-prone: two of the existing inline computations used a slightly different lineCount formula (last.LineIndex - first.LineIndex + 1 vs. last.LineIndex + last.LineCount - first.LineIndex), making it hard to know which was correct.
• Hard to audit: every copy had to be read independently to verify correctness.
• Missing a safety net: there was no enforcement of the documented ordering requirement that first must start at or before last.

Similarly, ConvertPureCSharpExpressionValue and ComputeValueSource each had duplicate condition arms calling the same UnwrapValueChildrenToTokens fallback, and GetAttributeSource had two identical loops over token children for HtmlAttributeValueIntermediateNode and CSharpExpressionAttributeValueIntermediateNode.

What

• Extracted MergeSourceSpans(first, last): a single, documented internal static helper that merges two ordered SourceSpan values into one covering span, using the correct formula for length, lineCount, and endCharacterIndex. Replaces all six inline span-merge expressions across the three resolver files.
• Added Debug.Assert: enforces the precondition that first.AbsoluteIndex <= last.AbsoluteIndex, catching ordering bugs immediately in debug builds.
• Deduplicated condition branches:
  • Collapsed the two duplicate UnwrapValueChildrenToTokens fallback arms in ComputeValueSource (bound-string + dynamic vs. complex/non-dynamic) into a single else branch.
  • Removed the redundant inner else chains in ConvertPureCSharpExpressionValue, using an early return so the UnwrapValueChildrenToTokens fallback only appears once.
  • Merged the two identical child-token loops in GetAttributeSource for HtmlAttributeValueIntermediateNode and CSharpExpressionAttributeValueIntermediateNode into a single pattern-match branch.
• Added unit tests (DefaultTagHelperResolutionPhaseTest.cs): 5 tests covering the MergeSourceSpans helper — same-line merge, multi-line merge, adjacent spans, identical span passed for both arguments, and null file path preservation.