[Fusion] Reduce metadb row reads in the result merge hot path

[michaelstaib]

No description provided.

[Copilot]

Pull request overview

Reduces metadata DB row reads on the Fusion result-merge hot path by enabling single-row retrieval patterns and in-place meta row updates that preserve parent pointers.

Changes:

• Added APIs to fetch the underlying parsed/meta row once (SourceResultElement/Document) to avoid repeated reads for token type + range.
• Introduced ReplacePreserveParent to overwrite meta rows in-place without re-reading the parent pointer.
• Added TryGetProperty overloads that return both the value element and its Selection to avoid extra selection resolution work in ValueCompletion.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/HotChocolate/Fusion/src/Fusion.Execution/Text/Json/SourceResultElement.cs	Adds GetValueRow() to expose the parsed row for reuse.
src/HotChocolate/Fusion/src/Fusion.Execution/Text/Json/SourceResultDocument.Text.cs	Adds GetValueRow(Cursor) helper for callers to reuse a single row read.
src/HotChocolate/Fusion/src/Fusion.Execution/Text/Json/CompositeResultElement.cs	Adds internal TryGetProperty overload returning Selection.
src/HotChocolate/Fusion/src/Fusion.Execution/Text/Json/CompositeResultDocument.TryGetProperty.cs	Adds TryGetNamedPropertyValue overload returning Selection alongside the value.
src/HotChocolate/Fusion/src/Fusion.Execution/Text/Json/CompositeResultDocument.MetaDb.cs	Adds ReplacePreserveParent fast-path overwrite method.
src/HotChocolate/Fusion/src/Fusion.Execution/Text/Json/CompositeResultDocument.cs	Switches assignments to use ReplacePreserveParent and source row reads.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Results/ValueCompletion.cs	Uses new property lookup overload to reuse Selection and reduce meta reads.

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