Support for escaping curly braces in embedded expression

[bleaphar]

This pull request introduces support for escaped character sequences in HTTP syntax parsing, enabling the handling of complex scenarios such as escaped curly braces within variable values. Key changes include the addition of a new syntax node for escaped characters, updates to parsing logic, and corresponding tests.

Parsing Enhancements:

• Added HttpEscapedCharacterSequenceNode to represent escaped character sequences in the syntax tree. This node trims escape characters (`This pull request introduces support for escaped character sequences in HTTP syntax parsing, enabling the handling of complex scenarios such as escaped curly braces within variable values. Key changes include the addition of a new syntax node for escaped characters, updates to parsing logic, and corresponding tests.

Parsing Enhancements:

) from the text. (src/Microsoft.DotNet.Interactive.Http.Parsing/Parsing/HttpEscapedCharacterSequenceNode.cs)

• Updated HttpRequestParser to detect and parse escaped character sequences using the new ParseEscapedCharacterSequence method. (src/Microsoft.DotNet.Interactive.Http.Parsing/Parsing/HttpRequestParser.cs) [1] [2] [3] [4]

Syntax Tree Updates:

• Modified HttpVariableValueNode to support adding HttpEscapedCharacterSequenceNode as a child node. (src/Microsoft.DotNet.Interactive.Http.Parsing/Parsing/HttpVariableValueNode.cs)
• Enhanced HttpRootSyntaxNode to process escaped character sequences when resolving variable values, ensuring proper handling of escaped braces. (src/Microsoft.DotNet.Interactive.Http.Parsing/Parsing/HttpRootSyntaxNode.cs)

Testing:

• Added unit tests to validate support for escaped curly braces and triple-escaped curly braces in variable values. (src/Microsoft.DotNet.Interactive.Http.Tests/ParserTests.Variables.cs)

[shyamnamboodiripad]

Is the escaping only intended to work on the RHS of variable assignment? Do we support literal values inside embedded expression within the body of requests for instance, and can those literal values contain {{ sequences that may need to be escaped?

(Sorry drawing a blank on how the embedded expressions are supposed to work within body and whether they only support references to other variables etc. :) If literal values are not supported within embedded expressions and only other variable references are supported there in, then feel free to ignore...)

[jonsequitur]

Did you intend to repeat this section?

[jonsequitur]

It might not be necessary to add so many cases. The goal of these tests is generally not to test every detail of the feature. The unit tests already cover that. They tend to be useful for catching issues with different combinations of syntax (e.g. does the parser move correctly from one node type to another) as well as incomplete syntax (e.g. does the parser handle it gracefully when the user is typing and the syntax is malformed in various normal ways.)