Fix JsonNode.GetPath() to properly escape characters in property names

[Copilot]

Description

JsonNode.GetPath() produces invalid JSON Path syntax when property names contain characters requiring escaping. Two issues:

1. Property names starting with $ (e.g., $defs, $ref) used dot notation ($.$defs) instead of bracket notation ($['$defs'])
2. Single quotes and backslashes in property names were not escaped in bracket notation

Before:

JsonNode.Parse("""{"$defs":{"foo['bar":"baz"}}""")["$defs"]["foo['bar"].GetPath();
// Returns: $.$defs['foo['bar']  (invalid JSON Path)

After:

// Returns: $['$defs']['foo[\'bar']  (valid JSON Path)

Changes:

• Add $ to special characters triggering bracket notation in JsonReaderHelper.cs
• Add AppendEscapedPropertyName helper methods that escape ' → \' and \ → \\, optimized using IndexOfAny to append prefix slices directly instead of per-character iteration
• Add StringBuilder.Append(ReadOnlySpan<char>) polyfill in Common/src/System/Text/StringBuilderExtensions.cs for .NET Standard 2.0 and .NET Framework support
• Update JsonObject.GetPath, WriteStack.AppendPropertyName, and ReadStack.AppendPropertyName to use escaping
• Update test expectations to reflect corrected JSON Path syntax

Customer Impact

Users receiving invalid JSON Path strings from GetPath() that cannot be parsed or used with JSON Path tools.

Regression

No. This behavior has existed since the feature was introduced.

Testing

• Added new test cases covering $-prefixed names, single quotes, backslashes, and combinations
• Added reproduction case from issue
• All 49,808 System.Text.Json tests pass

Risk

Low. The change makes output spec-compliant. Existing consumers expecting the old (invalid) format may need adjustment.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>JsonNode.GetPath() doesn't consider character escaping</issue_title>
<issue_description>### Description

The JsonNode.GetPath() generally does well for C#-valid property names, but any string is valid as a key in JSON, and when a key has characters that need escaping, nothing happens and you get an invalid (unparsable) JSON Path as a result.

Reproduction Steps

JsonNode.Parse("""{"$defs":{"foo['bar":"baz"}}""")["$defs"]["foo['bar"].GetPath();

Expected behavior

Returns $.$defs['foo[\'bar'].

Returns $['$defs']['foo[\'bar'].

• The single quote needs to be escaped.
• $defs is not valid as a shorthand property name, so it needs to be in bracketed syntax.

Actual behavior

Returns $.$defs['foo['bar'].

Regression?

Unknown

Known Workarounds

None.

Configuration

Seems to happen everywhere. dotnetfiddle, Linux.

Will update with more details in a comment.

Other information

Originally reported json-everything/json-everything#406.
</issue_description>

Comments on the Issue (you are @copilot in this section)

@ Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis See info in [area-owners.md](https://github.com/dotnet/runtime/blob/main/docs/area-owners.md) if you want to be subscribed.

Issue Details

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Description

The JsonNode.GetPath() generally does well for C#-valid property names, but any string is valid as a key in JSON, and when a key has characters that need escaping, nothing happens and you get an invalid (unparsable) JSON Path as a result.

Reproduction Steps

JsonNode.Parse("""{"$defs":{"foo['bar":"baz"}}""")["$defs"]["foo['bar"].GetPath();

Expected behavior

Returns $.$defs['foo[\'bar'].

Actual behavior

Returns $.$defs['foo['bar'].

Regression?

Unknown

Known Workarounds

None.

Configuration

Seems to happen everywhere. dotnetfiddle, Linux.

Will update with more details in a comment.

Other information

Originally reported json-everything/json-everything#406.

Author:	gregsdennis
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

• Fixes JsonNode.GetPath() doesn't consider character escaping #83547

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[Copilot]

Pull request overview

This PR fixes JsonNode.GetPath() to produce valid JSON Path syntax when property names contain special characters requiring escaping. The fix addresses two specific issues: (1) property names starting with $ now use bracket notation instead of invalid dot notation, and (2) single quotes and backslashes in property names are now properly escaped within bracket notation.

Key Changes:

• Added $ to the set of special characters that trigger bracket notation in JSON Path
• Implemented AppendEscapedPropertyName helper methods to escape ' as \' and \ as \\ within bracket notation
• Updated all JSON Path generation sites (JsonObject.GetPath, WriteStack.AppendPropertyName, ReadStack.AppendPropertyName) to use the new escaping

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
JsonReaderHelper.cs	Added $ to special characters set; implemented two overloads of AppendEscapedPropertyName for escaping quotes and backslashes
JsonObject.cs	Updated GetPath to call AppendEscapedPropertyName when using bracket notation
WriteStack.cs	Updated AppendPropertyName to call AppendEscapedPropertyName when using bracket notation
ReadStack.cs	Updated AppendPropertyName to call AppendEscapedPropertyName when using bracket notation
ParentPathRootTests.cs	Added comprehensive test cases covering $-prefixed names, quotes, backslashes, and nested scenarios
PolymorphicTests.CustomTypeHierarchies.cs	Updated test expectations from dot notation (e.g., $.$type) to bracket notation (e.g., $['$type'])
ReferenceHandlerTests.Deserialize.cs	Updated test expectations from dot notation to bracket notation for $-prefixed metadata properties
ConstructorTests.Exceptions.cs	Updated test expectations from dot notation to bracket notation for $-prefixed properties